Comment 0 for bug 887563

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