Reusing segments after hitting the max segments per cell limit

Starts https://github.com/numenta/nupic.core/issues/232.

Have yet to implement logic for reusing synapses, will do that separately.
Okay, I tried using a return parameter, but it seemed to slow things down rather than speed them up. Maybe I'm doing it wrong. But I propose that we punt on this, and come back to it if we profile it and its a bottleneck.