updatestreamctl uses event_streamer (WIP)
unify return type for EventStreamer
Big leap: update_stream uses binlog_streamer

The update stream API also changes. It accepts a group id
based BinlogPosition as input, and streams events one at a
time. The position is also a separate event at the end of
each transaction.
fix typo in event_streamer_test
Merge branch 'master' into replication