The actual code for `Server` was previously organized very confusingly:
it was thrice layered with `SpawnAll` and `Serve` which both appeared in
conn.rs despite not having anything to do with the lower-level conn API.
This commit changes that, removing all layering and having the code for
the higher-level `Server` appear inside `server.rs` only.

`server.rs` is currently littered with `cfg`s for `http1` or `http2`,
since the majority of server behaviour is only available with either one
of those feature flags active. This is hard to maintain and confusing to
read, so this commit extracts the two implementations into their own
files, since there is very little benefit in sharing code between the
two.

- Since the `if` condition already causes the loop to `break`,
  the `else` is not necessary. We wouldn't have reached the `else`
  block, anyway, if the prior `if` condition passed.
- We are more likely to poll a successful chunk than finish
  the request or throw an error. Thus, it is best if we go
  for the optimistic route and check for the successful
  case first.

Expose local address of tcp connection in AddrStream.

Closes #2773

Libcurl expects that headers are iterated in the same order that they
are recieved. Previously this caused curl tests 580 and 581 to fail.
This necessitated exposing a way to preserve the original ordering of
http headers.

SUMMARY OF CHANGES: Add a new data structure called OriginalHeaderOrder that
represents the order in which headers originally appear in a HTTP
message. This datastructure is `Vec<(Headername, multimap-index)>`.
This vector is ordered by the order which headers were recieved.
Add the following ffi functions:
- ffi::client::hyper_clientconn_options_set_preserve_header_order : An
     ffi interface to configure a connection to preserve header order.
- ffi::client::hyper_clientconn_options_set_preserve_header_case : An
     ffi interface to configure a connection to preserve header case.
- Add a new option to ParseContext, and Conn::State called `preserve_header_order`.
  This option, and all the code paths it creates are behind the `ffi`
  feature flag. This should not change performance of response parsing for
  non-ffi users.

Closes #2780

BREAKING CHANGE: hyper_clientconn_options_new no longer
  sets the http1_preserve_header_case connection option by default.
  Users should now call
  hyper_clientconn_options_set_preserve_header_case
  if they desire that functionality.

Signed-off-by: cuishuang <imcusg@gmail.com>

The previous commit broke this, but it wasn't released, so released versions will never notice the breakage.

…s` (#2817)

* refactor: use `matches` macro to flatten code

* refactor: prefer `matches` over explicit matching

* refactor: reuse `can_write_body` method

* refactor: use `matches` over `if`-`let`

* refactor: nested `if`-`else` as early return

* refactor: move inner `match` logic outside

* refactor: unneeded `return` in `match`

* refactor: remove unneeded reference matching

* refactor: use early returns for idle check

* refactor: use `matches` macro for Boolean `match`

HEAD responses should not have content-length implicitly set by hyper.

Co-authored-by: Jannes Timm <jannes@cloudflare.com>

)

This allows setting the HTTP/2 `SETTINGS_MAX_HEADER_LIST_SIZE` which advertises to the peer the maximum header size allowed, and internally is enforced.

Closes #2826

Signed-off-by: r <ryanrussell@users.noreply.github.com>