Issue #219 - I noticed the number of async tasks kept increasing.  It was calling asyncTaskStarted in beginReplication, however it's not doing this on iOS.  Fix accordingly.

After removing asyncTaskStarted from beginReplication, I noticed errors related to shutting down the reqeustExecutor.  Instead of shutting it down, do as iOS does and just keep references to requests (futures in this case) and cancel them when needed.

https://github.com/couchbase/couchbase-lite-android/issues/219