Reduce save-draft timeout to 10 seconds

We've become more confident about saving drafts (although issues
remain ensure old ones are deleted as new ones take their place).
I'm dropping the timeout to 10 seconds to reduce network activity
and the number of discarded drafts generated while writing them, which
fills up the users' trash.
Reduce IMAP timeouts to reduce network load

We've received some criticism of being overly aggressive with our
IMAP timeouts, in particular keepalive traffic.