Cleanup the last_merge logic. There are two reasons for clearing
last_merge when we are dealing with integrity, and these are:

o Clear when handing the request to the driver, so we don't merge on
  a started request.

o Clear when a request is taken off the list. This cannot be done from
  the driver (above case would already have been hit), but it can
  happen
  when we merge two requests.

This makes it a lot nicer, it was always peculiar how we cleared in
put_request.