binlog server: dead (kindof)

The binlog server functionality has now become part
of the update stream. It's available as a StreamKeyrange
funciton. The update stream continues to support the
ServeUpdateStream function like before.
The new StreamKeyrange function has been futurized
by separating the filtering logic into a standalone function.
It also streams one transaction per response. This simplifies
the clients.
The binlog_player has been rewritten to use the new
StreamKeyrange rpc. Since the StreamKeyrange function
only communicates group_id and server_id, a good chunk of
_vt.blp_checkpoint is now obsolete. We should consider
dropping that table in favor of a more suitable one.
For now, I've changed all the code neither update nor
use the binlog file or position. This is to ensure that
it doesn't get accidentally relied upon.
The binlog player has also been simplified to not do
any additional filtering. We want all filtering to be
done in one place (server side) to avoid confusion.
Next up:
- Delete all the dead code
- Move binlog streaming into its own package
- Move new structs into proto
- Revisit logging and debug/vars
- Undo google mysql patch