MongoClient libmongoc resource management

Modeled the persistent connection and Resource management after:

 * https://github.com/PocketRent/hhvm-pgsql
 * https://github.com/facebook/hhvm/blob/master/hphp/runtime/ext/mysql/mysql_common.cpp
 * https://github.com/facebook/hhvm/blob/master/hphp/runtime/ext/mysql/ext_mysqli.cpp