try builds should form an actual queue

[jdm]

When homu receives a request for a try build, it tries to fulfill it as soon as possible. This means that any builds that have not yet started for an existing try build (because there are no builders available) will never start, therefore starving the existing try build in favour of the new one.

[Manishearth]

This should be an option; Rust uses Travis for tryamd doesn't have the same starvation issues