Split client_channel into client_channel & child_channel.

Safely managing disconnection and goaways needs reference counting at the child channel level, which was near impossible to provide with the previous embedding of the child channel in the client channel.

This is a (hopefully) no-op refactoring to provide that split. The next CL in this series will actually get disconnection and goaway somewhat right.
	Change on 2014/12/12 by ctiller <ctiller@google.com>
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Created by MOE: http://code.google.com/p/moe-java
MOE_MIGRATED_REVID=81985590