Another attempt to alleviate nodejs/build#3014

debugger: decrease timeout used to wait for the port to be free

By default, the debugger would query the specified inspector
sever port to see if it's available before starting the server,
and it would keep retrying until a timeout (previously 9999 ms)
is reached. This timeout seems to be longer than necessary. This
patch decreases the timeout to 3 seconds.

debugger: end connections used to check availability of the port

The debugger check the availability of the specified inspector server
port by trying to connect to it, and retry if the connection can be
established (which means the port is not yet free), but it never ends
those connections. This patch adds code to end the connections after
they are established to avoid taking up more resources than necessary.

test: increase timeout in the debugger tests on Windows

On certain Windows machines it may take longer for the testing
processes to receive outputs from the debugger process. Increasing
the timeout to 15 seconds to avoid failing the tests in those
cases.