Versioning parallel builds

[pazdera]

When there are two people building the same package at the same time, the versions will clash, because they're are only committed at the end of each build.

(reported by @tombettany)

Maybe we could lock packages when building them, so two people wouldn't be able to build a package at the same time.

[pazdera]

This was done in the dr-ci (link) branch that I never got around to finishing. If someone could cherry-pick that out, that'd be really useful.