WIP-- Not Yet Compiling or Functional -- WIP

* No longer expose WT_SESSION into Erlang at all as WT's model is to
maintain one WT_SESSION per-thread and we don't know anything about
threads in Erlang.

* async_nif worker threads don't pull both mutexes on every loop
when processing requests, only one

* async_nif provides a worker_id (int, 0 - MAX_WORKERS) within the
work block scope which we use to find our per-worker WT_SESSIONs

* async_nif maintained a number of globals which I'm moving into
the NIF's priv_data so that on upgrade/reload we have a fighting
chance to "Do the Right Thing(TM)".

* NIF Upgrades/Reloads started to plumb this in.

* Use a khash to manage the cache of URI->WT_CURSORs per WT_SESSION.

* Added start/stop positions into truncate call to allow for truncating
sub-ranges data.

* Lots of other details I'm sure I've forgotten and more left undone.
Search for "TODO:" or try to compile to see what's left, and then
there is a need for a lot more tests given all this new complexity.