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.

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