Register new builds in a single transaction.

Registering an evaluation can cause a large number of write queries using
individual transactions. This is really sub-optimal as commiting a transaction
will often result in a database sync to the disk.

* src/cuirass/database.scm (db-add-build): Do not create a new transaction for
each new build, instead ...
(db-register-builds): ..., create a single transaction per evaluation
registration.