* Makes it possible to safely call acquire_lock so that it throws an
exception instead of returning false if it can't get a lock (which
most existing uses assumed it already does).
* Fix some omissions from the requirelockingbeforewrite option (it
now checks on delete).
* Modinfo uses a versioned cache, so it is not necessary to delete
items, only increase the version. (Provided we keep track of
cacherev carefully...)
sam marshall
0f8577784d