(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
It's important that all public functions that will become part of our ABI are properly namespaced. Otherwise you can run into symbol collisions with other libraries.

Consider adding a `void *user_data` parameter for convenience in the callback.