Should only be concerned with current/future roombookings and events

There appears no point in making roombookings or events in the past.

Limit retrieved TermTime roombookings and activities, and Booker bookings to state.get('now') which defaults to the current datetime but can be overridden in configuration files (as a ISO8901 Zulu datetime).

This should hopefully reduce the pull of large numbers of bookings from Booker.