cuirass: Store new information in database to prepare new HTTP API

integration.

* bin/evaluate.in (fill-job): New procedure.
(main): Use it to fill informations (nix-name, system) that will later be
added to database.
* doc/cuirass.texi (Database)[Derivation]: Add system and nix_name fields.
(Database)[Builds]: Add id, status, timestamp, starttime and stoptime
fields. Remove output field.
(Database)[Outputs]: New table describing the build outputs.
* src/cuirass/base.scm (build-packages): Add new fields to build object before
adding it to database.
* src/cuirass/database.scm (db-get-build, db-get-builds): New procedures to get
a build by id from database and a list of builds using filter parameters
respectively.
* src/schema.sql (Outputs) : New table.
(Derivations): Add system and nix_name columns.
(Builds): Remove output column and add id, status, timestamp, starttime and
stoptime columns.