database: Add a Checkouts table.

It is used to know when a new evaluation must be triggered and to display
input changes.

* Makefile.am (dist_sql_DATA): Add 'src/sql/upgrade-3.sql'.
* bin/cuirass.in (main): Call DB-SET-EVALUATION-DONE at startup to clear
'in-progress' evaluations.
* bin/evaluate.in (input-checkout, format-checkouts): Rename '#:name' to
'#:input'.
* doc/cuirass.texi (Stamps): Remove section.
(Checkouts): New section.
* src/cuirass/base.scm (fetch-input, fetch-inputs, compile-checkouts): Rename
'#:name' to '#:input'.
(evaluate): Remove the COMMITS argument.  Add an EVAL-ID argument.  Don't call
DB-ADD-EVALUATION because it was called sooner.  Remove the EVAL-ID argument
to AUGMENT-JOB because it's a closure.
(build-packages): Add an EVAL-ID argument.  Call DB-SET-EVALUATION-DONE once
all the derivations are registered.
(process-specs): Replace the stamping mechanism by the primary key constraint
of the Checkouts table: call "evaluate" only when DB-ADD-EVALUATION is true,
which means that at least one checkout was added.  Change the EVALUATE and
BUILD-PACKAGES arguments accordingly.
* src/cuirass/database.scm (db-add-stamp, db-get-stamp): Remove procedures.
(db-set-evaluations-done, db-set-evaluation-done): New exported procedure.
(db-add-checkout): New procedure that returns #f if a checkout with the same
revision already exists.
(db-add-evaluation): Replace the EVAL argument with a SPEC-NAME and a
CHECKOUTS arguments.  Insert the evaluation only if at least one checkout was
inserted.  Return #f otherwise.
(db-get-checkouts): New procedure.
(db-get-evaluations, db-get-evaluations-build-summary): Handle the
'in_progress' column, remove the 'commits' column.  Return the result of
DB-GET-CHECKOUTS as part of the evaluation.
* src/cuirass/templates.scm (input-changes, evaluation-badges): New
procedures.
(evaluation-info-table): Rename "Commits" to "Input changes".  Use
INPUT-CHANGES to display the input changes that triggered the evaluation.  Use
EVALUATION-BADGES to display a message indicating that the evaluation is in
progress.
* src/schema.sql (Stamps): Remove table.
(Checkouts): New table.
(Evaluations): Remove the 'commits' column.  Add an 'in_progress' column.
* src/sql/upgrade-3.sql: New file with SQL queries to upgrade the database.
* tests/database.scm (make-dummy-eval): Remove procedure.
(make-dummy-checkouts): New procedure.
("sqlite-exec"): Remove the 'commits' column.  Add the 'in_progress' column.
("db-update-build-status!", "db-get-builds", "db-get-pending-derivations"):
Update the arguments of DB-ADD-EVALUATION accordingly.
* tests/http.scm (hash-table=?): Add support for lists of hash tables.
(evaluations-query-result): Replace '#:commits' with '#:checkouts'.  Return a
list instead of returning one element, for symmetry.
("fill-db"): Add a new input so that the second checkout can refer to it.
Replace EVALUATION1 and EVALUATION2 with CHECKOUTS1 and CHECKOUTS2.  Update
the arguments of DB-ADD-EVALUATION accordingly.
("/api/queue?nr=100"): Take the CAR of the EVALUATIONS-QUERY-RESULT list to
make it symmetrical with the other argument of HASH-TABLE=?.