(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
I would probably avoid the global/default callbacks altogether and allow them to be assigned on a `mongoc_client_t`. Additionally that would mean that `mongoc_client_pool_t` would need a similar function so it can be applied when it creates a new `mongoc_client_t`.