Add async support

[SGrondin]

TrollDB is so fast, sometimes I get a response before I have time to send my query, so it would be nice if you added async support (ie with callbacks).

Otherwise, great job, we love it and use it in prod.

[dermariusz]

It seems that TrollDB already has support for that feature because the API is by default non blocking and every function of the API accepts additional callbacks. It's probably just undocumented.