- Re-worked the prev_record_reads() function to return the lower bound of
  number of different table access scans that will be performed.


mysql-test/r/join.result:
  BUG#14940: testcase
mysql-test/t/join.test:
  BUG#14940: testcase
sql/sql_select.cc:
  BUG#14940: Slow join order is chosen: 
  - Re-worked the prev_record_reads() function to return the lower bound of
    number of different table access scans that will be performed.
sql/sql_select.h:
  BUG#14940: Slow join order is chosen:
  - Added comments in struct POSITION
  - Added POSITION::ref_depend_map: bitmap of tables that the table access
    method depends on.