Fix some bugs around waiting for worker threads to end during SQL slave stop.
Free Log_event after parallel execution (still needs to be made thread-safe by
using rpl_group_info rather than rli).
Wait for all worker threads to finish when stopping the SQL thread.
(Only a basic wait; this still needs to be fixed to include timeout
logic as in sql_slave_killed()).
Hook in the wait-for-prior-commit logic (not really tested yet).
Clean up some resource maintenance around rpl_group_info (may still be some
smaller issues there though).
Add a ToDo list at the top of rpl_parallel.cc
