Remove workers hash table.

* src/cuirass/database.scm (db-add-worker): Rename ...
(db-add-or-update-worker): ... into this new procedure.
(db-get-worker, db-remove-unresponsive-workers): New procedures.
* src/cuirass/remote-server.scm (%workers): Remove it.
(pop-build): Adapt it.
(remove-unresponsive-workers!): Remove it.
(read-worker-exp): Adapt it.
(zmq-start-proxy): Ditto.
* tests/database.scm ("db-add-worker"): Rename ...
("db-add-or-update-worker"): ... into this new test.
("db-get-worker", "db-remove-unresponsive-workers"): New tests.