Explicit quit and ping timeout behave the same way,
but reattach after abandoning/losing the previous session
(without the break being detected server-side) is more aggressive
about replaying missed messages, at the cost of potential duplication.

(So that the registration burst displays correctly)

during nick change, removeInternal(client) was being called even before checking
whether the new nick was in use or reserved. Reproduction steps:

1. Log in a client 'alice'
2. Log in a client 'bob'
3. bob issues /nick alice, which fails (correctly) with:
:oragono.test 433 bob alice :Nickname is already in use
4. alice issues /msg bob hi, which fails (incorrectly) with:
:oragono.test 401 alice bob :No such nick

* Clients are now renamed, not disconnected, on reservation timeout
* Nick reservation config is now its own subsection