* src/sql/upgrade-4.sql: New file.
* Makefile.am (dist_sql_DATA): Add it.
* src/schema.sql (Specifications)[period]: New field.
* src/cuirass/base.scm (process-specs): Honor the specification period.
* src/cuirass/database.scm (db-get-time-since-previous-build): Rename it into ...
(db-get-time-since-previous-eval): ... this new procedure.
* src/cuirass/specification.scm (<specification>)[period]: New field.
(specification->sexp, sexp->specification): Adapt accordingly.
* src/cuirass/templates.scm (specification-edit): Ditto.
* tests/database.scm ("db-get-time-since-previous-build"): Remame it into ...
("db-get-time-since-previous-eval"): ... this new procedure.