https://jira.hyperledger.org/browse/FAB-2033



The user facing documentation refers to channels, but the vast majority
of the fabric code refers to chains.  This CR switches the protos to
refer to channels as they are generally the interface between the user
and fabric code.

Change-Id: I869e6d68c1228e638a99e2460274889f18f2543d
Signed-off-by: Jason Yellick <jyellick@us.ibm.com>