Add parallel building

[notriddle]

Instead of batching stuff together, run every item in the backlog immediately with everything ahead of it merged in. This has a couple nice side-effects:

• It reduces the time taken from O(E*log(N)) to O(E).
• It results in a history that doesn't have arbitrary octopus merges.

On the other hand...

• It increases the amount of builds run from O(E*log(N)) to O(E*N)
• The time decrease only applies if you have an arbitrarily high number of build nodes. If you run out of nodes, it jumps to O(N) again.

So we do want to provide both.

[Macarse]

Here's a nice diagram explanation of the algorithm: https://docs.openstack.org/infra/zuul/gating.html#testing-in-parallel

[notriddle]

I should probably mention, just for completeness's sake, that this is going to be awhile.

Making good on parallel building requires near-bottomless build capacity and the ability for bors to insta-cancel builds. Travis limits free projects to something like five concurrent builds, which would be enough for bors-ng/bors-ng itself, but isn't going to cut it for rust-lang/rust.

Maybe we could use Travis's "auto-cancel" feature to drop known-bad builds from the queue? That could work, but I'd rather have something where bors can proactively throw errors if it's misconfigured. Being easy to set up matters.