(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
So, the idea is that it might make sense to be able to set either the global observer table or a client-specific server table as a user.  Then when the callback trigger is hit internally, we can check whether a callback table is set on this client, and if so use it, otherwise fall back to using the global table?