Rebuild less

Two main changes:
- track when we last checked a PR, and don't look at it again until its last updated date is later than the internally tracked last_checked date (on startup, all PRs are checked)
- when looking for a build on jenkins, consider any build with matching parameters -- running or not (except when we were asked to PLS REBUILD)

review by @bantonsson (the second change should fix the second problem that was plaguing us, as suggested by you)