Get code completions asynchronously

[Boddlnagg]

Currently on every keystroke racer.exe (and depending on configuration also rustc --print sysroot) is run synchronously to fetch code completions, leading to delays while typing. Racer should be invoked asynchronously instead, if possible.

See also #251 for some prior discussion.

[Vbif]

Maybe we need swich to RLS first

[xilec]

RLS is in pre-alpha stage now. I suppose it is too early time for switching to RLS.