Implement PhutilPHPObjectProtocolChannel and other channel changes

Summary:
See D2614. This is a small step toward providing some primitives to make "arc" (and other commands) use a more client/server architecture. See also D2841.

This adds four features to Channels:

• PhutilPHPObjectProtocolChannel, a protocol channel which transmits PHP objects as messages.
• PhutilSocketChannel::createNewPair(), which creates a pair of connected socket channels for testing.
• PhutilChannel::flush(), which blocks until writes complete (also useful for testing).
• Allows PhutilSocketChannel to accept separate read/write sockets (primarily useful for creating a socket channel on stdin/stdout).

Test Plan: Unit tests.

Reviewers: vrana

Reviewed By: vrana

CC: aran

Differential Revision: https://secure.phabricator.com/D2836