Add machine field to Worker table.

* src/sql/upgrade-2.sql: New file.
* Makefile.am (dist_sql_DATA): Add it.
* src/schema.sql (Workers): Add "machine field".
* src/cuirass/database.scm (db-get-builds): Return "worker" field.
(db-add-worker): Honor new "machine" field.
(db-get-workers): Ditto.
* src/cuirass/remote-worker.scm (remote-worker): Adapt it.
* src/cuirass/remote.scm (<worker>)[machine]: New field.
(worker-machine): New procedure.
(worker->sexp, sexp->worker): Adapt accordingly.
(generate-worker-name): Ditto.
* tests/database.scm (%dummy-worker): Add "machine" field.