This commit aims to centralize all of the terminal interactions
throughout sbt. It also seeks to hide the jline implementation details
and only expose the apis that sbt needs for interacting with the
terminal.

In general, we should be able to assume that the terminal is in
canonical (line buffered) mode with echo enabled. To switch to raw mode
or to enable/disable echo, there are apis: Terminal.withRawSystemIn and
Terminal.withEcho that take a thunk as parameter to ensure that the
terminal is reset back to the canonical mode afterwards.

The ask user thread is a background thread so it's fine for it to block
on System.in. By blocking rather than polling, the cpu utilization of
sbt drops to 0 on idle. We have to explicitly handle <ctrl+d> if we
block though because the JLine console reader will return null both if
the input stream returns -1

When the user inputs `run` or `runMain` we shouldn't print the warning
about multiple classes because in the case of run they already will be
prompted to select the classes and in the case of runMain, they are
already required to specify the class name.

Bonus:
 * improve punctuation
 * add clear screen to selector dialogue
 * print selector dialogue in one call to println -- this should prevent
   the possibility of messages from other threads being interlaced with
   the dialogue

There typically are fewer than 10 main classes in a project so allow the
user to just input a single digit in those cases. Otherwise fallback to
a line reader.

It's a bit annoying to have to hit enter here.

Also, this should fix #5162 because if
there is no System.in attached, the read will return -1 which will cause
sbt to quit.

Prior to this change, if a network command came in, it would run in the
background with no real feedback in the server ui. Prior to this change,
running compile from the thin client would look like:

sbt:scala-compile>
[success] Total time: 1 s, completed Dec 12, 2019, 7:24:43 PM
sbt:scala-compile>

Now it looks like:
sbt:scala-compile>
[info] Running remote command: compile
[success] Total time: 1 s, completed Dec 12, 2019, 7:26:17 PM
sbt:scala-compile>

Presently if a server command comes in while in the shell, the client
output can appear on the same line as the command prompt and the command
prompt will not appear again until the user hits enter. This is a
confusing ux. For example, if I start an sbt server and type
the partial command "comp" and then start up a client and run the clean
command followed by a compile, the output looks like:

[info] sbt server started at local:///Users/ethanatkins/.sbt/1.0/server/51cfad3281b3a8a1820a/sock
sbt:scala-compile> comp[info] new client connected: network-1
[success] Total time: 0 s, completed Dec 12, 2019, 7:23:24 PM
[success] Total time: 0 s, completed Dec 12, 2019, 7:23:27 PM
[success] Total time: 2 s, completed Dec 12, 2019, 7:23:31 PM

Now, if I type "ile\n", I get:
[info] sbt server started at local:///Users/ethanatkins/.sbt/1.0/server/51cfad3281b3a8a1820a/sock
ile
[success] Total time: 0 s, completed Dec 12, 2019, 7:23:34 PM
sbt:scala-compile>

Following the same set of inputs after this change, I get:
[info] sbt server started at local:///Users/ethanatkins/.sbt/1.0/server/51cfad3281b3a8a1820a/sock
sbt:scala-compile> comp
[info] new client connected: network-1
[success] Total time: 0 s, completed Dec 12, 2019, 7:25:58 PM
sbt:scala-compile> comp
[success] Total time: 0 s, completed Dec 12, 2019, 7:26:14 PM
sbt:scala-compile> comp
[success] Total time: 1 s, completed Dec 12, 2019, 7:26:17 PM
sbt:scala-compile> compile
[success] Total time: 0 s, completed Dec 12, 2019, 7:26:19 PM
sbt:scala-compile>

To implement this change, I added the redraw() method to LineReader
which is a wrapper around ConsoleReader.drawLine; ConsoleReader.flush().
We invoke LineReader.redraw whenever the ConsoleChannel receives a
ConsolePromptEvent and there is a running thread.

To prevent log lines from being appended to the prompt line, in the
CommandExchange we print a newline character whenever a new command is
received from the network or a network client connects and we believe
that there is an active prompt.

It is better that sbt not expose the implementation detail that
LineReader is implemented by JLine. Other terminal related apis should
be handled by sbt.internal.util.Terminal.

To reduce the noise of supershell, this commit imposes a 10 millisecond
minimum duration before a task will appear in the supershell lines.

In order to make supershell work with println, this commit introduces a
virtual System.out to sbt. While sbt is running, we override the default
java.lang.System.out, java.lang.System.in, scala.Console.out and
scala.Console.in (unless the property `sbt.io.virtual` is set to
something other than true). When using virtual io, we buffer all of the
bytes that are written to System.out and Console.out until flush is
called. When flushing the output, we check if there are any progress
lines. If so, we interleave them with the new lines to print.

The flushing happens on a background thread so it should hopefully not
impede task progress.

This commit also adds logic for handling progress when the cursor is not
all the way to the left. We now track all of the bytes that have been
written since the last new line. Supershell will then calculate the
cursor position from those bytes* and move the cursor back to the
correct position. The motivation for this was to make the run command
work with supershell even when multiple main classes were specified.

* This might not be completely reliable if the string contains ansi
cursor movement characters.

We do not need a large blank zone with the virtual system.out.

This communicates intent better than clearScreen(0).

There is push to move away from complete sentences in warnings.

Also display invalid response character.