Group commit for MySQL slave committting to wsrep provider

[Seppo Jaakola]

Implement transaction bundling for MySQL slave committing into wsrep provider. This may help with the bottleneck of having only one MySQL slave facing commit time delay of synchronous provider.

[Seppo Jaakola (seppo-jaakola)]

This work goes to private branch for hot-fix builds only.

Fix has been committed where variable wsrep_mysql_replication_bundle controls how many replication events will be grouped together.

[Seppo Jaakola (seppo-jaakola)]

Replication events are grouped in SQL slave thread by skipping events which may cause commit. This way the wsrep node acting in MySQL slave role and all other wsrep nodes in provider replication group, will see same (huge) transactions.

Pay attention to following when testing this feature:
* mysql replication stream is not flushed. When load in MySQL master stops, you have to send at least wsrep_mysql_replication_bundle amount of void transactions to the master, to make sure your application payload was committed in wsrep provider cluster.

* max write set size limit is not taken in consideration. You should know your transactions and not set wsrep_mysql_replication_bundle too high. If max write set size is reached, wsrep provider will refuse to replicate the aggregated transaction.

[Seppo Jaakola (seppo-jaakola)]

This implementation is still experimental, but was merged in the development head, nevertheless. Slave group commit feature is configurable, and is off by default.
Merge to the development head happened with revision: http://bazaar.launchpad.net/~codership/codership-mysql/wsrep-5.5/revision/3809