Google semi-sync repl patch

[Arjen Lentz]

See http://code.google.com/p/google-mysql-tools/wiki/SemiSyncReplication

There's a version in a MySQL 6.0 (from code provided to MySQL), this could be backported to 5.1 (or even 5.0), or the original code extracted from the big Google patches again. Backporting might make more sense, if the 6.0 code is ok.

MySQL 6.0 specific:
  https://code.launchpad.net/~hezx/mysql-server/semi-sync-replication
  http://lists.mysql.com/commits/49059
Uses specific MySQL 6.0 plugin hooks, so using that code won't happen unless we backport the 6.0 replication plugin API (which may be feasible)

[Arjen Lentz (arjen-lentz)]

For re-extracting out of the big Google V2 patchset, the 5.0.37 patch (from Google V1) can be used as a template.
http://google-mysql-tools.googlecode.com/svn/trunk/mysql-patches/mysql-5.0.37_semisync.patch