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make Puller a subclass of ReplicationInternal

https://github.com/couchbase/couchbase-lite-android/issues/398
pullerinternal now kicks off changetracker

https://github.com/couchbase/couchbase-lite-android/issues/398
when changetracker finishes, replication stops

https://github.com/couchbase/couchbase-lite-android/issues/398
call start instead of beginReplicating

https://github.com/couchbase/couchbase-lite-android/issues/398