Remove MutexID list

[tiif]

Remove MutexID list in SynchronizationObjects to avoid memory leaks.

cc #3967

[tiif]

I feel I have been constantly running into already mutably borrowed: BorrowError when it comes to unblock_thread. It always takes me a while to realise that it has something to do with unblock_thread again.

Usually, the list of blocked thread ids is stored in a struct. Alternating between retrieving thread id (which involves a mutable borrow to the struct) and unblock thread will usually lead to BorrowMut error because the struct often need to be borrowed again during the unblock callback.

[RalfJung]

Generally you have to take great care of your RefCell handles. Never keep them any longer than you have to. For every function you call while holding the handle, ensure it doesn't reentrantly access the same data again.

If we didn't use RefCell, you'd get a borrow checking error due to changing a vector while iterating it. That would be nicer but unfortunately the reference patterns are too dynamic to make that work.

[tiif]

For every function you call while holding the handle, ensure it doesn't reentrantly access the same data again.

This is a pretty nice way to see it, thanks.

[tiif]

Aside from the questions in the comments, this is ready

@rustbot ready

[RalfJung]

Thanks, looks great overall! But there's a bunch of minor things.

[RalfJung]

@rustbot author

[tiif]

@rustbot ready

[RalfJung]

Just some final nits. :)
You can squash after taking care of those (with --keep-base as usual).

@rustbot author

[tiif]

@rustbot ready

[RalfJung]

Oh, wtf... as long as there is a "requested changes" review, it won't merge, even if the merge button is pressed by the person that requested the changes...