Each indexing session, for each index, will create a key-value heartbeat of the format:
[prefix, xid] -> [indexing-type, genesis time, heartbeat time]
Indexing session that are expected to be exclusive will throw an exception if another, active, session exists.

Motivation:

1. Represent the heartbeat in every index during multi target indexing (currently - only the master index has a sync lock)
2. Keep a heartbeat during mutual indexing, which can allow better automatic decision making
3. Decide about exclusiveness according to the indexing method (currently - user input).