SemanticDataLookup, use DISTINCT for non subject items, refs 2928, 3531

[mwjames]

This PR is made in reference to: #2928, #3531

This PR addresses or contains:

• Move SQLStore::fetchSemanticData to SemanticDataLookup #2928 caused a regression with the case described in EntityLookup->getPropertyValues( null, ..., limit=15 ) fails to return all property values #3531

This PR includes:

• Tests (unit/integration)
• CI build passed

Fixes #3531

[kghbln]

Back-ported with 9152f94

[kghbln]

Back-ported with 9152f94

I guess some of the previous tidies need to be backported too or this backport reverted. Not sure if this needs to be in 3.0.x

[mwjames]

I guess some of the previous tidies need to be backported too or this backport reverted.

Should be #3585.

[kghbln]

Should be #3585.

Thanks for the pointer. Done!