Merge branch 'release-v21.1' into develop
Added catch for undefined sourceResource in enrich-type.

sourceResource can be missing in items where the status is "deleted".
Added threading to enrich_records (#7282)
Add catch for incomplete read in database export (#7155)

Added handling of IncompleteRead exception in
export_database.store_result_into_file().  This may not resolve
the issue of closed HTTP connections to CouchDB, but may give us
more information as to what sometimes fails.
Fixed Google thumbnail image URL in test for hathi_identify_object

This image URL has apparently changed.
Bumped version number to 22.0
Merge branch 'release-v22.0'