Simplified some logic in CBLPuller

Traun pointed out that _caughtUp will always be NO at this point,
since -startChangeTracker is only called from -beginReplicating
after clearing _caughtUp.
Improved handling of CBL_Puller changesTotal count

This should fix the changesTotal temporarily going negative.
Also, if some incoming revisions already exist, it now responds by
increasing changesProcessed instead of lowering changesTotal, which
makes more sense.
Fixes #351

Conflicts:
	Source/CBL_Puller.m
Oops, forgot to push CocoaHTTPServer.