fastboot: add TCP protocol.

This CL implements a TCP protocol for use with fastboot. Protocol
description is given in fastboot_protocol.txt, some examples of
expected behavior can also be found in tcp_test.cpp.

Usage is:
  fastboot -s tcp:<hostname>[:port] <command>

Bug: http://b/26558551
Change-Id: If53a514a534489c617db32c4fea8819949121282