Sequence objects are implemented using special tables.
These tables do not have primary key and only one row.
NEXTVAL is basically update from existing value to new
value. In Galera this could mean that two write-sets
from different nodes do not conflict and this could
lead situation where write-sets are executed
concurrently and possibly in wrong seqno order.

This is fixed by using table-level exclusive key for
SEQUENCE updates. Note that this naturally works
correctly only if InnoDB storage engine is used for
sequence.

This fix does not contain a test case because while
it is possible to syncronize appliers using dbug_sync
it was too hard to syncronize MDL-lock requests to
exact objects. Testing done for this fix is documented
on MDEV.
Signed-off-by: Julius Goryavsky <julius.goryavsky@mariadb.com>