This considerably speeds up cascade deleting.

* src/sql/upgrade-9.sql: New file.
* Makefile.am (dist_sql_DATA): Add it.
* src/schema.sql (Jobs_build, BuildProducts_build, Notifications_build): New
indexes.

src/cuirass/base.scm (fetch-input): Add the commit timestamp to the returned
association list,
(process-specs): Pass a timestamp taken at procedure start and another one
taken after inputs are fetched to "db-add-evaluation" procedure. Once the
evaluation is over, call "db-set-evaluation-time" to save the evaluation
completion time.
src/cuirass/database.scm (db-set-evaluation-time): New procedure,
(db-add-checkout): Handle the "timestamp" field,
(db-add-evaluation): add "checkouttime" and "evaltime" arguments. Modify the
associated SQL query accordingly.
(db-get-builds): Use "Builds.timestamp" instead of "timestamp" as this field
is also part of the Evaluations table.
src/schema.sql (Checkouts): Add "timestamp" field,
(Evaluations): add "timestamp", "checkouttime" and "evaltime" fields.
src/sql/upgrade-9.sql: New file.
tests/database.scm (sqlite-exec): Adapt Evaluations table insertions to include
"timestamp", "checkouttime" and "evaltime" required fields.