LP#1733695: Pull record attribute values from multiple sources

LP#1733695: Pull record attribute values from multiple sources

Previously, a multi-valued record attribute could only be extracted using
one strategy (XPath, tag+subfield, fixed field, etc).  For multi-valued
attributes, though, there's no reason to restrict this as long as the values
from various sources would, after normalization, have the same shape.  So
now we allow that, and we update the item_lang definition in order to allow
searching for records with multiple languages in various boolean ways.

Signed-off-by: Mike Rylander <mrylander@gmail.com>