Auto merge of #3809 - RalfJung:fd-refcell, r=oli-obk

FD: remove big surrounding RefCell, simplify socketpair

A while ago, I added the big implicit RefCell for all file descriptions since it avoided interior mutability in `eventfd`. However, this requires us to hold the RefCell "lock" around the entire invocation of the `read`/`write` methods on an FD, which is not great. For instance, if an FD wants to update epoll notifications from inside its `read`/`write`, it is very crucial that the notification check does not end up accessing the FD itself. Such cycles, however, occur naturally:
- eventfd wants to update notifications for itself
- socketfd wants to update notifications on its "peer", which will in turn check *its* peer to see whether that buffer is empty -- and my peer's peer is myself.

This then also lets us simplify socketpair, which currently holds a weak reference to its peer *and* a weak reference to the peer's buffer -- that was previously needed precisely to avoid this issue.

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