Keep Connection and other objects in sync with Comm in closure callbacks

Keep Connection and other objects in sync with Comm in closure callbacks

There are lots of small bugs, inconsistencies, and other problems in
Connection closure handlers. It is not clear whether any of those
problems could result in serious runtime errors or leaks. In theory, the
rest of the code could neutralize their negative side effects. However,
even in that case, it was just a matter of time before the next bug will
bite us due to stale Connection::fd and such. These changes themselves
carry elevated risk, but I think we have to do them to get closer to a
reliable code as far as Connection maintenance is concerned; otherwise,
we will keep chasing their deadly side effects.

Long-term, all these manual efforts to keep things in sync should become
unnecessary with the introduction of appropriate Connection ownership
APIs that automatically maintain the corresponding environments (TODO).

Also marked a few newly uncovered bugs in the official code.