Use specification for periodic builds.

When the periodic argument is passed to a job, it will be registered only if
the time difference between the current time and the registration time of the
last build is greater than the specified period. Make sure that the last job
that is searched is part of the same specification.

* src/sql/upgrade-16.sql: New file.
* Makefile.am (dist_sql_DATA): Add it.
* src/schema.sql (Builds_job_name_timestamp): New index.
* src/cuirass/database.scm (db-get-time-since-previous-build): Add
"specification" argument,
(db-register-builds): pass it.