This will avoid many queries trying to figure out what the latest build status
is, which will hopefuly simplify queries as well as improving performance.
The Guix Build Coordinator doesn't record timestamps for events
currently. This is something to fix, but allow for build statuses without
timestamps as they're not necessary.
Allow for build status information to be submitted by POST request. This
required some changes to the builds and build_status tables, as for example,
the Cuirass build id may not be available, and the derivation may not be know
yet, so just record the derivation file name.
Christopher Baines