"mutex lock failed" during initial pull replication of large # of docs

https://github.com/couchbase/couchbase-lite-core/issues/813

Initial launch of freshly installed copy of the client app will perform a large continuous pull replication. Logging indicates this exception is reliably thrown after pulling the same document with multiple revisions. Subsequent launches of the app do not throw the exception and the replication will complete, however the checkpoint never advances. Further delete/reinstall/replicate cycles will always result in the same exception, and the same checkpoint. No warnings or errors are logged on the sync gateway or in the client.