This class wraps an existing Pazpar2 record command session as cached by a
Pazpar2 session ID in the HTTP session.
It's constructed to be used by the holdings plug-in to pick up holdings
from the Record.
The class relies on the caller to provide a HTTP session (containing the
Pazpar2 session ID), a Pazpar2 URL and a Record document from the most i
recent Pazpar2 request. Together they'd provide a consistent wrapper.