[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.
can the rush end date not be a different date?  or do we model a rush period which spans midnight as being two rush periods, one starting at midnight and the ohter ending at midnight?