core: add RemainAfterElapse= setting to timer units

Previously, after a timer unit elapsed we'd leave it around for good,
which has the nice benefit that starting a timer that shall trigger at a
specific point in time multiple times will only result in one trigger
instead of possibly many. With this change a new option
RemainAfterElapse= is added. It defaults to "true", to mimic the old
behaviour. If set to "false" timer units will be unloaded after they
elapsed. This is specifically useful for transient timer units.