LP#1731960-preserving book bag entries with new values during bib merge

LP#1731960-preserving book bag entries with new values during bib merge

The asset.merge_record_assets() removes bib records from containers when
merging and deleting the record as appropriate but bib containers with
the btype of 'bookbag' (used as reading lists) should instead be updated
to the new bib record. Includes Pgtap test.

Signed-off-by: Rogan Hamby <rhamby@equinoxinitiative.org>
Signed-off-by: Cesar Velez <cesar.velez@equinoxinitiative.org>
Conflicts:
	Open-ILS/src/sql/Pg/999.functions.global.sql