Second part of WL#7640, NDB$EPOCH2 and NDB$EPOCH2_TRANS algorithms.

These algorithms extend NDB$EPOCH with delete-delete handling via
'reflected' secondary operations.

They are also controlled by the ndb_slave_conflict_role server variable,
allowing PRIMARY/SECONDARY role changes without shutting down MySQLD.

Finally new counters are added to track use of the two algorithms, 
and on the secondary to show the number of refresh operations received,
and the number of reflected operations received, and discarded.

Ndb_conflict_refresh_op_count
Ndb_conflict_reflected_op_prepare_count
Ndb_conflict_reflected_op_discard_count

Existing tests are slightly modified.
New tests ndb_rpl_conflict_epoch2, ndb_rpl_conflict_epoch2_trans and 
ndb_rpl_conflict_epoch2_extra are added.