Workaround for truncated _changes feed responses

If TDSocketChangeTracker gets an unparseable response whose beginning
looks like the expected JSON, assume the socket got cut off by some
intermediary and go through the usual retry mechanism.
Fixes #241.
Fix memory leaks (reference cycles) in replicator

Fixes couchbase/couchbase-lite-ios#37

Conflicts:
	Source/CBLMultipartReader.h
	Source/CBL_Puller.m
	Source/CBL_Pusher.m