Added the ability to create channels from client_channel_factory. This
will be used by the grpclb code to create the channels to communicate
with the LB servers.

This extends the existing http parser to support requests as well as responses.

httpcli continues to exist and work as it has previously, though in the new
directory src/core/http (to reflect the fact the directory now contains code
relevant to parsing requests, which httpcli would not generally involve itself
in).