Use the writer worker for all write queries.

* .dir-locals.el: Add "with-queue-writer-worker".
* bin/cuirass.in: Modify "with-queue-writer-worker" scope to include the
web-server operations.
* src/cuirass/database.scm (with-db-writer-worker-thread): Export it.
(with-db-writer-worker-thread/force): New macro.
(db-add-input, db-add-checkout, db-add-specification, db-remove-specification,
db-add-evaluation, db-abort-pending-evaluations, db-set-evaluation-status,
db-set-evaluation-time, db-add-output, db-add-build-product, db-add-event,
db-delete-events-with-ids-<=-to): Use "with-db-writer-worker-thread" or
"with-db-writer-worker-thread/force" instead of "with-db-worker-thread".
* src/cuirass/metrics.scm (db-update-metrics): Ditto.
* tests/database.scm ("db-init"): Set "%db-writer-channel".
* tests/http.scm ("db-init"): Ditto.
* tests/metrics.scm ("db-init"): Ditto.