User defined logger callback implementation

[temeo]

Added static wsrep::log::logger_fn() method to allow user to
provide logger callback.

[mkaruza]

This look good, but in general how can we distinguish between logs coming from of database, wsrep-lib or galera? Maybe we can use prefix argument in log constructor to indicate that.

[temeo]

This look good, but in general how can we distinguish between logs coming from of database, wsrep-lib or galera? Maybe we can use prefix argument in log constructor to indicate that.

Opened a separate issue to address this: #16