Asynchronous dowloads of dependencies.

[bobeff]

This pull request is a work in progress intended to be a starting point for those who want to finish this feature. Currently, it is not working stably and random crashes, stalls, and other misbehaviors are observed caused by bugs in the asynctools library or in the Nim's standard library asyncdispatch module. Also, is left to be implemented a limit of maximum async downloads when downloading free (not locked) dependencies.