Partially fixed earlier in #467

now that we have client.destroyed again, we don't need to test the
"0 sessions && brb disabled" condition in both AddSession() and
destroy()

Use []uint32 in bitset instead of []uint64, because it's harder to guarantee
64-bit alignment of []uint64 than I had realized:

https://go101.org/article/memory-layout.html

Fixes #519

Quit messages aren't supposed to clobber each other under any
circumstances, but actually only the default "Connection closed"
message was non-clobbering, all others would clobber.

Also ensure that people can remove their own persistent modes via CS AMODE

Fixes #505

Setting `closed` on the socket (which can mean either "the socket is broken"
or "we should close the socket at the next opportunity") was racing against
the final write. Even if socket.closed is true, we should attempt to send
buffered message data to the socket, before we send the `finalData` and
actually call `Close`.

If you were banned and the ban was only detected when you proxied
(because you were proxying from a DLINE'd IP), you'd get an incorrect
quit message: `QUIT: Bad or unauthorized PROXY command`. This propagates
the correct ban message as the quit line.

Some clients seem to automatically poll for channel state