TPAC bucket item retrieval operates in streaming mode

TPAC bucket item retrieval operates in streaming mode

TPAC bucket item retreive fleshes bib records with large blobs of MARC
data.  When a bucket contains a few thousand items, the size of the
data passed around in atomic retreival mode will exceed the typical jabber
max stanza size and result in a failure.  Retrieve the records in
streaming mode instead.

Signed-off-by: Bill Erickson <berick@esilibrary.com>
Signed-off-by: Ben Shum <bshum@biblio.org>