binlog_streamer: bug fix for skipping events

The binlog streamer sometimes reads incomplete events due to a race
between the writer and reader. In such cases, it obtains a new position
but fails to read the corresponding event. If this happens, then a
relaunch skips the event because the last position was already recorded.
In this fix, we separate out what we've seen from what we've
transmitted, and always restart mysqlbinlog based on what we've
transmitted, which is always at a transaction boundary.
binlog streamer: handle file not found on rotate

Sometimes the new file is not ready yet. So, retry once after a delay to
prevent raising of unnecessary alarms.
mysqlbinlog info log change

Redirect mysqlbinlog output to info log instead of error or warning.
Those 'error' messages are business as usual, not really errors.