2i improvements added in 1.4.4-1.4.6

* Add 2i term regex filter
* Make 2i pagination sort optional
* Use sext NIF (C implementation)
* Fix vnode sending > max_results items for 2i query
* Add 2i AAE command
* Fix some mixed cluster queries (1.3.x and 1.4.2)
* Make memory backend obey 2i return terms properly

/cc @jtuple @russelldb This is a port of all the 2i changes for 1.4.4 err 1.4.5 no wait 1.4.6.
They passed all the updated 2i tests in develop except for verify_2i_mixed_cluster, as mixed clusters are broken in develop currently. I will link the riak_test PR soon.