Implement Ndb_slave_max_replicated_epoch status variable

New status variable, initialised on Slave start from ndb_apply_status
table contents and maintained as 'reflected' ndb_apply_status WRITE_ROW 
events are applied.  ndb_apply_status events logged by the Slave server,
or any server in the IGNORE_SERVER_IDs list are used.

The status variable is reset when RESET SLAVE is executed.

Additionally :
 - Ndb handler code accessing the MasterInfo structure is factored out into
   new files (ndb_mi.h, ndb_mi.cc)
 - The master_epoch member of MasterInfo is no longer used/maintained.

Two new testcases added to verify status variable initialisation and
maintenance.

This variable is used in a future patch series to detect distributed update 
serialisation violations (conflicts).