database: Make 'register-items' transactional.

database: Make 'register-items' transactional.

* guix/store/database.scm (SQLITE_BUSY, register-output-sql): New variables.
(add-references): Don't try finalizing after each use, only after all the
uses (otherwise a finalized statement would be used if #:cache? was #f).
(call-with-transaction): New procedure.
(register-items): Use call-with-transaction to prevent broken intermediate
states from being visible.

* .dir-locals.el (call-with-transaction): indent it.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>