On error when fetching stats, ignore and return whatever you have so far.
Add a comment and remove an mutext unlock on error because this function is not
where the lock is acquired.
* Check to make sure that all ASYNC_NIF_REPLY() calls are at the end of their
blocks, just before return and after releasing any local resources or locks.

* Check the return value of when setting up cursor caches so as not to miss an
ENOMEM or other error.

*Cleanup and free resources when closing a connection handle.

* Add a few missing mutex unlock calls on error paths.

* Ensure all resources are free'd/released/closed during truncate error paths.

* Free up alloc'ed copies of keys, cursor handles and sessions on unload.