refactoring of inboundfinder to make it reusable for b2ghaystack

I also added a dependency to 'concurrent.futures' to speedup the retrieval of builds information - it is used in b2ghaystack for now (https://bugzilla.mozilla.org/show_bug.cgi?id=1003793).

If the pull request is accepted, we could rewrite b2ghaystack to reuse B2GBuildsFinder and remove some of the logic duplication in this code. I will be happy to create the bug if I can and mark it as a good first bug, as I think it is trivial enough.