Processor: Acquire locks before executing an LU

If we're running in a "new style" LU we may need some locks, as required
by the ExpandNames function, to be able to run. We'll walk up the lock
levels present in the needed_locks dictionary and acquire them, then run
the actual LU. LUs can release some or all the acquired locks, if they
want, before terminating, provided they update their needed_locks
dictionary appropriately, so that we know not to release a level if they
have already done so.

Reviewed-by: iustinp