Fixed an issue with revision properties during a validation callback

CBLRevision object passed to callback didn't have _id or _rev properties
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
Updated CBForest