adb: fix adb client running out of sockets on Windows

Background
==========

On Windows, if you run "adb shell exit" in a loop in two windows,
eventually the adb client will be unable to connect to the adb server. I
think connect() is returning WSAEADDRINUSE: "Only one usage of each
socket address (protocol/network address/port) is normally permitted.
(10048)". The Windows System Event Log may also show Event 4227, Tcpip.
Netstat output is filled with:

  # for the adb server
  TCP    127.0.0.1:5037         127.0.0.1:65523        TIME_WAIT
  # for the adb client
  TCP    127.0.0.1:65523        127.0.0.1:5037         TIME_WAIT

The error probably means that the client is running out of free
address:port pairs.

The first netstat line is unavoidable, but the second line exists
because the adb client is not waiting for orderly/graceful shutdown of
the socket, and that is apparently required on Windows to get rid of the
second line. For more info, see
https://github.com/CompareAndSwap/SocketCloseTest

 .

This is exacerbated by the fact that "adb shell exit" makes 4 socket
connections to the adb server: 1) host:version, 2) host:features, 3)
host:version (again), 4) shell:exit. Also exacerbating is the fact that
the adb protocol is length-prefixed so the client typically does not
have to 'read() until zero' which effectively waits for orderly/graceful
shutdown.

The Fix
=======

Introduce a function, ReadOrderlyShutdown(), that should be called in
the adb client to wait for the server to close its socket, before
closing the client socket.

I reviewed all code where the adb client makes a connection to the adb
server and added ReadOrderlyShutdown() when it made sense. I wasn't able
to add it to the following:

* interactive_shell: this doesn't matter because this is interactive and
  thus can't be run fast enough to use up ports.
* adb sideload: I couldn't get enough test coverage and I don't think
  this is being called frequently enough to be a problem.
* send_shell_command, backup, adb_connect_command, adb shell, adb
  exec-out, install_multiple_app, adb_send_emulator_command: These
  already wait for server socket shutdown since they already call
  recv() until zero.
* restore, adb exec-in: protocol design can't have the server close
  first.
* adb start-server: no fd is actually returned
* create_local_service_socket, local_connect_arbitrary_ports,
  connect_device: probably called rarely enough not to be a problem.

Also in this change
===================

* Clarify comments in when adb_shutdown() is called before exit().
* add some missing adb_close() in adb sideload.
* Fixup error handling and comments in adb_send_emulator_command().
* Make SyncConnection::SendQuit return a success boolean.
* Add unittest for adb emu kill command. This gets code coverage over
  this very careful piece of code.

Change-Id: Iad0b1336f5b74186af2cd35f7ea827d0fa77a17c
Signed-off-by: Spencer Low <CompareAndSwap@gmail.com>