Fix problems stopping a replicator

The -stop method can end up being called re-entrantly, and it wasn’t
expecting that, so Bad Stuff could happen.

Also made the CBLReplicator’s -stop method simply clear the batcher
instead of flushing it, since any queued changes in it shouldn’t be
processed anyway (they’d result in remote requests that would just
immediately be stopped.)

For #187, #191, #193