Rewrite the entire internal signal handling/wakeup.

We now solve things in a much less racy way, sending SIGUSR1 internally,
which is blocked except in poll/epoll. This should be more stable,
require no more fixed-length wakeups (no more wakeup every 20 ms in
each server), and upset Valgrind less.

However, it's also complex to get right, so it might introduce new bugs.