Allow running multiple merge builds in parallel

[Kobzol]

When performing merges against distinct target branches, they could be executing in parallel, either on the same test branch (if possible to do using GitHub API) or in several test branches.

[Rhythm1710]

If this is not assigned yet, Can I work on this?

[Kobzol]

Well, given that bors currently doesn't have any merge functionality, I imagine that it would be quite hard to parallelize it 😆 There are several issues in this repo that describe "nice to have" functionality, but a lot of the base stuff is still missing. I'll try to clean up the issues in the upcoming days.

[Rhythm1710]

Okay, got it!