(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
Need to add checks on the passed-in table, a la [libbson](https://github.com/mongodb/libbson/blob/master/src/bson/bson-memory.c#L278-L283)