Add repl_bucket_types check that RTQ is drained after bucket type mis-match

During RT, in the case of a bucket type mis-match on the sink, riak_repl2_sink_conn was not acknowledging the replication with the source RTQ after dropping the put, resulting in the queue filling. 

Add a check to make sure that the queue is empty after a dropped put due to bucket type mismatch.

Issue: https://github.com/basho/riak_repl/issues/611