(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
This is always a point of contention for me. Total microoptimization (that is actually different performance on every architecture) versus removing the if conditional check everywhere. Both are completely reasonable, as long as they are consistent.