LP#1749475 Make context variable more explicit

LP#1749475 Make context variable more explicit

We are dealing in this area with two potential contexts: the context
of the holdings, and the context of the event to process the request.

Let's be a little more explicit by using $event_context_org rather than
$context_org for the second, and $holdings_context_org for the first.

Signed-off-by: Dan Wells <dbw2@calvin.edu>