Update bookings first and handle conflicts

Do updates before creations, and don't stop if a conflict is encountered. Subsequent syncs should perform conflicting actions and resolve outstanding conflicts.

Also required adding a booker.cancel_events operation move blocking events (deleted from TermTime). The intention is that the normal schedule would call both operations cancel_events update_events (in that order). Several safety measures in the operation to limit potential damage.

Closes: #38 (closed)