works like
https://github.com/devinus/poolboy
but with these features added
promise_call/2, promise_call/3, promise_transaction/2, promise_transaction/3
- while a poolboy worker firstime checkout a poolboy worker, a new worker is created.
- while the last process checkouted by a poolboy worker checkin, the worker checkedin is destroyed.
- this feature makes async_poolboy:transaction able to use inside poolboy worker.
rebar3 ct