- Untested changes to the FS use of the DLM:

- Untested changes to the FS use of the DLM:
 - if we already have a lock that covers the extent, use it
 - if we need a PR lock and already have a PW lock, use the PW lock
 - if we can promote a PR lock to a PW lock and be covered, do the conversion
- keep track of the readers and writers using the lock
- fixed a lock conversion bug (we were ignoring locks held by other members of
  the conversion list when checking for granted compat)
- Brought today's changes onto the branch

Next up: fixing the dlm locking mess.