Only MyISAM tables locked with LOCK TABLES ... WRITE were affected.

A query that is optimized with index_merge doesn't reflect rows
inserted within LOCK TABLES.

MyISAM doesn't flush a state within LOCK TABLES. index_merge
optimization creates a copy of the handler, which thus gets
outdated MyISAM state.

New handler->clone() method is introduced to fix this problem.
For non-MyISAM storage engines it allocates a handler and opens
it with ha_open(). For MyISAM it additionally copies MyISAM state
pointer to cloned handler.


mysql-test/r/index_merge.result:
  A test case for bug#20256.
mysql-test/t/index_merge.test:
  A test case for bug#20256.
sql/ha_myisam.cc:
  clone method added to handler class.
sql/ha_myisam.h:
  clone method added to handler class.
sql/handler.cc:
  clone method added to handler class.
sql/handler.h:
  clone method added to handler class.
sql/opt_range.cc:
  Use handler clone method.