Safe Lock Acquisition Should Specifically Target Different Lock Modes

[James Coleman]

Currently safely_acquire_lock_for_table always acquires an AccessExclusiveLock, which is generally safe in the sense that:

• It's always sufficient for the operation since it's the highest lock type for a table.
• Most of the places we use it need that high of a lock level anyway.

However, it has a few disadvantages:

• There are some existing places where we don't need that high of a lock level (albeit the operations are still fast, so it hasn't seemed like a big deal).
• We could use it in places where we explicitly do not need a high level lock.

If we change this, we'd need t make sure that nesting calls (or even calls in the same transaction -- even though we discourage that) can't upgrade locks since that's could result in a deadlock.