1. Fix index access costs for MERGE tables, set block_size=myisam_block_size/#underlying_tables 
   instead of 0 which it was before.
2. Make index scans on MERGE table to return records in (key_tuple, merge_table_rowid) order, 
   instead of just (key_tuple) order. This makes an index scan on MERGE table to be truly a ROR-scan
   which is a requirement for index_merge union/intersection.


myisammrg/myrg_queue.c:
  BUG#17314: Make index scans on MERGE table return records ordered by (keytuple, merge_table_rowid).
mysql-test/r/index_merge.result:
  Testcase for BUG#17314
mysql-test/r/merge.result:
  BUG#17314: update testcase result
mysql-test/t/index_merge.test:
  Testcase for BUG#17314
sql/ha_myisammrg.cc:
  BUG#17314: For MERGE tables, set handler::block_size to myisam_block_size/#underlying_tables, and not to 0.