Improve locking safety for RID handles builds

[lawnjelly]

Additional locks are added. This is primarily to cover a potential race condition where the pool is resized from another thread during a get operation.

Notes

• This is compiled out of regular builds (so has no effect in vanilla Godot)
• It improves the thread safety of rids=handles and rids=tracked_handles builds.
• Only noticed this as we were discussing the handles builds in random getornull error spammed in console #60406.
• To my knowledge, these race condition haven't occurred for me in 5 months of use, but it is probably a good idea to close these possibilities.
• Luckily it doesn't seem to have much effect on performance adding the extra lock on the get. Godot 4 puts this lock on RID gets as well, probably for similar reasons.

Note that by design in Godot 3.x, the RID objects themselves aren't thread safe. The RID database itself is intended to be thread safe, but there's nothing to stop one thread e.g. deleting a RID object mid way while another thread is accessing that object.

[akien-mga]

Thanks!