Combined pull request for wsrep-API#19, wsrep-API#20, wsrep-API#22

[temeo]

This pull request implements changes required for codership/wsrep-API#19 and codership/wsrep-API#20, codership/wsrep-API#22

• Added application context for wsrep_sst_request_cb_t
• Added wsrep_trx_meta_t for commit_ordre_enter() and commit_order_leave()
• Renamed struct wsrep to struct wsrep_st

Do not merge yet, for review only now.