Scheduler: add daily events

[T-101]

Extend Scheduler plugin to run commands daily

[progval]

@daily name HH:MM:SS <command> is equivalent to @repeat --delay [seconds HH:MM:SS] name [seconds 1d] <command>, and shares its main issue: the event runs every 86400 seconds instead of every day, which does not behave as expected in presence of DST and leap seconds.

In order to address this, you need to add a new type of scheduler events for events which happens on a fixed clock (every X days, monthly, yearly, ...) and compute the next tick every time.

(Also, avoid loops to compute time, it uses more CPU and is more error prone, eg. this one does not terminate when a user provides 12:00:60 as time_of_day)