daemon: Drop our own scheduler and use sched

daemon: Drop our own scheduler and use sched

Our own implementation was trying to be very accurate which
comes with a large overhead and is largely unnecessary.

This patch removes the threading approach which kept multiple
timers and replaces it with one simple deadline scheduler.

Plugins will have to ensure that they are not blocking the
collection.

Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>