Add support for binlog_checksum in MariaDB.

When binlog_checksum is enabled, it changes the binlog format in a
backwards-incompatible way. Even though we don't currently verify
checksums, we need to know how to strip them and how to signal that
we understand the new format. Otherwise a server with checksums
enabled will refuse to send us binlogs.
Merge pull request #87 from youtube/mariadb

Add support for binlog_checksum in MariaDB.