RSU implementation

[Seppo Jaakola]

Implement rolling schema upgrade support in mysql side.

Online schema upgrade (OSU) is a feature enabling schema changes while cluster is online. With this enhancement, the cluster will have two separate OSU methods:
* total order isolation (TOI)
* rolling schema upgrade (RSU)

A little bit of design:

1. A new option variable must be introduced to choose between TOI and RSU methods
2. DDL processing must switch between TOI and RSU methods
3. RSU method must call wsrep->desync() and wsrep->resync() before and after the DDL processing
4. RSU method should also set THD's wsrep_on to 0 during DDL phase, this is convenience for the caller

A sibling task: https://bugs.launchpad.net/codership-mysql/+bug/887564 implements functionality to dictate whole cluster in STATEMENT replication during the OSU process. This may turn out inevitable act.

[Seppo Jaakola (seppo-jaakola)]

Intermediate implementation is pushed in wsrep-5.5 change set: http://bazaar.launchpad.net/~codership/codership-mysql/wsrep-5.5/revision/3609