Use wb_terms table for label lookup.

The default TermLookup should hit the wb_terms table instead of
loading full entities.

NOTE: EntityTermLookup is currently uncached. Several quick SQL
queries still seem better than loading the same number of full
entity blobs from memcached. See T74309 for thoughts on caching
terms.

DEPLOYMENT: Keep an eye on DB load, wb_terms table in particular.
Also note that this change should reduce memcached i/o significantly.

Bug: T72767
Change-Id: Ib16da3dcd3bb1f3da1aac9c4a732c7b891756371
Merge "Use wb_terms table for label lookup."