Query async

[dominicletz]

New function in Server that allows submitting jobs async. Initially this was a GenServer.cast, but I've added batching, so that if many async queries are pending they will be submitted in a single sqlite transaction. Which depending on your sqlite journaling mode can have significant performance impacts.

[coveralls]

Coverage decreased (-6.4%) to 86.818% when pulling 14fffd6 on diodechain:query_async into 1d47a6b on elixir-sqlite:master.

[ConnorRigby]

Thanks! this looks like a useful feature.
Would you mind adding a test?

[dominicletz]

Yeah, I wanted to get rid of my own fork with those changes, so I created a bunch of pulls today. I'm bit test-lazy, but give me a couple of days and i might do it