http: Change the paramater format from two-elements lists to pairs.

* src/cuirass/database.scm (assqx-ref): Remove exported procedure.
(db-get-builds, db-get-build): Adapt to new format.
* src/cuirass/http.scm (request-parameters): Use (cons key param) instead
of (list key param).
(url-handler): Adapt to new format.
* tests/database.scm ("db-get-builds"): Idem.