This reduces delays of zero-duration timers from up to 1 second to
zero.  libavahi-client schedules zero-duration timers often.

... and make TimerEvent a type alias for FineTimerEvent (i.e. swap
names).

If an InjectEvent callback schedules a timer, the loop will restart,
but the "busy" flag is still false.  The fix is to move the "again"
check before the "busy" setting.

Fixes use-after-free bugs causing assertion failures at shutdown,
because all "abandoned" SocketEvents are still in the linked list.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/986
Closes https://github.com/MusicPlayerDaemon/MPD/issues/987

Not only those which are "ready".

Prepare for adding more code here.

This implements proper rounding, amending commit dcbb9fe0

Similar to commits 1686f4e8 and
30a5dd26

Fixes the !HAVE_THREADED_EVENT_LOOP build.

Not for MPD, but for other applications which might want to copy its
event loop, but do not need multi-threading.

Instead of using this as a base class implementing a virtual method,
the new class IdleEvent can be used as a variable, decoupling
IdleMonitor's internal state from the derived class.

This is similar to commit 30a5dd26
which refactored TimeoutMonitor to TimerEvent.

By making each SocketMonitor keep track of its ready flags, this
removes a lot of overhead from EventLoop::RemoveFD().