(WIP) CDRIVER-400 Add a customizable client callback table

Basic skeleton for a customizable callback table, primarily for use by the PHP driver.  Some things:

-need to figure out what callbacks we need
-need to figure out best places/multiple places to include triggers
-I'm sure I've massacred the style on this
Probably no need for `on_` in the names. Typically that word is used by consumers to indicate they are attaching to an event callback. Up to you though.