Fix uncommon Open311 errors on processing updates.

Fixes #677. The query to fetch the related alert sometimes:

    * retrieves no result, causing an error on $alert->id
    * retrieves multiple results, causing deprecation warnings
    (e.g. which may at some point become future errors in DBIC)

This commit turns the resultset('DB::Alert')->find(...) into a
search(...) and loops over the results, to suppress them.