- Removes useless call to wsrep_xid_init() in wsrep_apply_events().
Transaction's xid is already initialized at that point.
- Adds call to wsrep_set_SE_checkpoint() for committing TOI events
in the applier side.
- Includes test case that reproduced the issue.
1. factored XID-related functions to a separate wsrep_xid.cc unit.
2. refactored them to take refrences instead of pointers where appropriate
3. implemented wsrep_get/set_SE_position to take wsrep_uuid_t and wsrep_seqno_t instead of XID
4. call wsrep_set_SE_position() in wsrep_sst_received() to reinitialize SE checkpoint after SST was received, avoid assert() in setting code by first checking current position.
Annotate_rows event needs to be preserved until the last Rows event has
been applied because after it has been applied thd->query points to the
query stored inside this event.
