utils: Change critical section terminology to worker threads.

As far as I'm aware, it's necessary to use a separate thread for interacting
with SQLite as one of the threads used for fibers will be blocked while the
SQLite query is running.

This doesn't mean all queries have to be executed one at a time though,
providing the queries are executed outside the threads used by fibers, and a
single connection isn't used in multiple threads.

These changes start to move in this direction, first by just changing the
terminology.

* src/cuirass/base.scm (clear-build-queue, cancel-old-builds): Change
with-db-critical-section to with-db-worker-thread.
* src/cuirass/database.scm (with-db-critical-section): Rename syntax rule to
with-db-worker-thread.
(db-add-input, db-add-checkout, db-add-specification, db-remove-specification,
db-get-inputs, db-get-specification, db-add-evaluation,
db-set-evaluations-done, db-set-evaluation-done, db-add-derivation-output,
db-add-build, db-update-build-status!, db-get-output, db-get-outputs,
db-get-builds-by-search, db-get-builds, db-get-build derivation-or-id,
db-add-event, db-get-events, db-delete-events-with-ids-<=-to,
db-get-pending-derivations, db-get-checkouts, db-get-evaluations,
db-get-evaluations-build-summary, db-get-evaluations-id-max,
db-get-evaluation-summary, db-get-builds-query-min, db-get-builds-query-max,
db-get-builds-min, db-get-builds-max, db-get-evaluation-specification): Change
from using with-db-critical-section to
with-db-worker-thread.
(with-database): Change syntax rule to use make-worker-thread-channel,
renaming from make-critical-section.
* src/cuirass/utils.scm (%critical-section-args): Rename parameter to
%worker-thread-args.
(make-critical-section): Rename to make-worker-thread-channel, and adjust
parameter and docstring.
(call-with-critical-section): Rename to call-with-worker-thread and adjust
parameter.
(with-critical-section): Rename to with-worker-thread, and adjust to call
call-with-worker-thread.
* tests/database.scm (db-init): Use make-worker-thread-channel rather than
make-critical-section.
* tests/http.scm (db-init): Use make-worker-thread-channel rather than
make-critical-section.