The timer channel may already have been drained by `case <-timer.C`. In
this case, we need not to drain it again or we'll block forever.
To fix this, stop draining the timer channel. Since we're not going to
use the timer again anyway, it should be fine.
The session manager has been upgraded to deal with reconnections.
Each session has its own expiration timer. Each time a request is
received, the expiration timer is reset.
A session can be closed (this is used when the user wants to logout).
When the IMAP connection is closed by the server, it's set to nil in the
session. The next time an IMAP command needs to be issued, the
connection is re-established.
Closes: https://todo.sr.ht/~sircmpwn/koushin/30
