Make async_scan scan each opened directory asynchronously

[mauriciopoppe]

This is an implementation of the comment in #236 (comment), pasting the algorithm for future reference:

local function async_scan(context, callback /* new */)
  for every path in context.paths_to_load
    async_scan(path) ->
        scan.scan_dir_async(parent) -> 
        wait for on_exit callback ->  increase counter and call `callback` when all the paths in context_paths are done

[cseickel]

Oh cool, you completely dropped plenary's scan_dir from this. I'll give this a try and merge it if there are no problems. I do have a lot of other things up in the air though so this might sit a bit, now that we know this is not the primary slowdown.

[cseickel]

Looks good, thanks!