Bug#12969454 - SET REASONABLE LIMITS ON SLAVE_* PARAMETERS RELATED TO MTS
Bug#13062867 - VALGRIND FAILURE IN RPL.RPL_CIRCULAR_FOR_4_HOSTS

The first bug is the main sub-case of Bug#12969454.
The max value for @@global.slave_checkpoint_group is reduced to become 
the max bytes in BLOB field (see mysql.slave_worker_info table) times 8.
The three more MTS configuration parameters maximum does not need any more refinements.
Anyway sys_vars suite tests are edited.

The issue with Bug#13062867 appears to deal with timing of reaching a state by Coordinator
where an extra warning is printed out. The worker errors out correctly as specified by 
the test's logics.