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
First step of splitting out part of Relay_log_info, so that different event groups being applied in parallel can each use their own copy.
First very rough sketch. We spawn and retire a pool of slave threads. Test main.alias works, most likely not much else does.
