Create a reaper to schedule the second phase of drop index for when the index's 'death' timestamp becomes checkpointed.

The catalog should have 1st and 2nd phase index drop functions. The 1st phase should call into the reaper with information to identify the index and a timestamp at which to start the 2nd phase. The reaper should call into the catalog to execute the second phase on the index information.

The reaper will use the functionality built in SERVER-38128 to be alerted when the 'death' timestamp becomes checkpointed.