Make sure that all registration queries are done from within a single database
worker. Otherwise, when builds from multiple evaluations are registered at
the same time, some contention occurs communicating with workers.

* src/cuirass/base.scm (new-outputs?, build-packages): Move build registration
to ...
* src/cuirass/database.scm (db-register-builds): ... this new procedure.
(with-db-worker-thread-no-timeout): New procedure. Use it in
"db-register-builds" to avoid timeout messages.