add testRetryQueueDeadlock() which reproduces severe issue

https://github.com/couchbase/couchbase-lite-android/issues/398
first pass at passing testRetryQueueDeadlock()

https://github.com/couchbase/couchbase-lite-android/issues/398
RemoteRequestRetry no longer implements Runnable

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