Allow OpenPsi to react on events

[stellarspot]

In current implementation OpenPsi is run in a loop where it checks if its rules are satisfied in the AtomSpace.

The proposal is to allow to subscribe OpenPsi on some events so if there are no new events it does nothing and when an event arrives it checks if the rules are satisfied.

There are 2 advantages of the event driven approach for OpenPsi:

• it does not waste CPU when there are no new events for some period of time
• it reacts immediately when an event arrives instead of waiting the minimum time between loop calls

[linas]

FYI, related work: See also opencog/atomspace#1750