Issue #58 - changeTracker might be null, depending on how quickly one calls goOffline() after kicking off replicator, so check for null first.  It's a race condition.

https://github.com/couchbase/couchbase-lite-java-core/issues/58
Issue #58 - add updateActive() and setError() methods, and use them when appropriate to match iOS behavior.  Also change a call from batcher.flush() to batcher.clear().

https://github.com/couchbase/couchbase-lite-java-core/issues/58