Add (gnu store database).

* guix/config.scm.in (%store-database-directory): New variable.
* guix/store/database.scm: New file.
* tests/store-database.scm: New file.
* Makefile.am (STORE_MODULES): New variable.
(MODULES, MODULES_NOT_COMPILED): Adjust accordingly.
(SCM_TESTS) [HAVE_GUILE_SQLITE3]: Add tests/store-database.scm.

Co-authored-by: Ludovic Courtès <ludo@gnu.org>