Refactor builds/build_requests tables to remove duplicate rows in builds table.

[catlee]

If you have a build that has merged more than one build request, you'll have one row in the builds table per build request. Each row will be identical, except for the column linking back to the build request.

This makes writing tools a bit complicated since you have to cope with the fact that more than one row in builds can correspond to the same single build on a slave.

I think we need another table that maps builds to build requests, e.g. a list of (build request id, build id) tuples. We could then drop the 'brid' column from the builds table.

[dustin]

This refactor should wait until sqlalchemy has landed in 0.8.4

[dustin]

schema change -> 0.8.5

[dustin]

The builds table will get dropped when builds actually move into the database, so I don't want to try do do anything in the interim.