database: ensure update-or-insert is run within a transaction
update-or-insert can break if an insert occurs between when it decides whether
to update or insert and when it actually performs that operation. Putting the
check and the update/insert operation in the same transaction ensures that the
update/insert will only succeed if no other write has occurred in the middle.
* guix/store/database.scm (call-with-savepoint): new procedure.
(update-or-insert): use call-with-savepoint to ensure the read and the
insert/update occur within the same transaction.