Processor: Move LU execution to its own method

This makes the try...finally code simplier, and helps adding a more
complex locking structure before the actual execution. It also fixes a
concurrency bug caused by the fact that write_count was read before
acquiring the BGL, and thus spurious config update hooks run could have
been triggered. This doesn't solve the issue of running config update
hooks for concurrent LUs.

Reviewed-by: iustinp