MWL#116: Efficient group commit: Fix bug that binlog pos stored by XtraDB during commit was wrong when more than one commit in group.

Now the actual binlog position for each commit is stored in THD, and XtraDB
can fetch the correct value from within commit_ordered() or commit().


mysql-test/r/group_commit_binlog_pos.result:
  Test case for XtraDB binlog position.
mysql-test/t/group_commit_binlog_pos-master.opt:
  Test case for XtraDB binlog position.
mysql-test/t/group_commit_binlog_pos.test:
  Test case for XtraDB binlog position.
sql/log.cc:
  Save binlog position corresponding to commit in THD, and make accessible to storage engine.
sql/sql_parse.cc:
  Add generic crash point for use in test cases.
storage/xtradb/handler/ha_innodb.cc:
  Update to use new method of getting current binlog position that works with group commit.
storage/xtradb/handler/ha_innodb.h:
  Update to use new method of getting current binlog position that works with group commit.