Proposal: New behavior for moving clock ticks

[bijanvakili]

Proposal: Remove the clock_tick context manager

This would require Session.flush() to automatically handle incrementing clock ticks for all instances in a session should a change be detected. This removes the need for using the context manager on individual entities which can sometimes be cumbersome.

[multigl]

Updates:

1. Activity -- require set before changing properties, expire on .commit
2. Explore
   a. each property change is a new clock tick, each correlated with either timestamp or activity
   b. all property changes are on the same clock tick, expired on .commit

[multigl]

#29