Add ndb_slave_last_conflict_epoch status variable

The ndb_slave_last_conflict_epoch status variable records the last slave
epoch which included conflict handling activity.

It is set when a MySQLD slave is applying binlogs to a cluster, and
detects a conflict.

It is set once the conflict causing transaction has been committed into
the slave cluster.

It is set to the slave cluster epoch assigned to the committed conflict 
causing transaction.

The intention is that in an asymmetric active-active setup (e.g. 
NDB$EPOCH), this will indicate when the PRIMARY cluster last experienced
a conflict.

If this variable is <= the MaximumReplicatedEpoch then from the Primary 
cluster's point of view there are no in-flight realignment operations, 
and therefore no outstanding realignment dependencies.

This can be a valuable state to be aware of when considering role
switchovers or failure handling steps required.

A new testcase is added to verify the behaviour of this new status
variable.