[buteo-syncfw] Introduce external sync settings.

This commit introduces settings to the scheduler that allow a external
process to take over the sync completely or to take over the sync while on
the rush period, this can be useful in case like always-up-to-date syncs.
Not at the moment, there's no way to set that afaik, rush only have a start and a end time, no date. Rush can start in next days, but should always end in the same day it started.