FTS - Apply RBAC only for target collections in a multi-collection index

Today, any query against a multi collection index is authenticated against all the source collections.

This may not be a very effective model for supporting the multi-tenant capability of the FTS multi-collection index.

RBAC rules could be slightly changed in this case,

1 - Blanket query (no target collections in query)

Authenticate against all source collections in the index definition.

2 - Collection scoped/targetted query.

Authenticate against only the target collections in the query.

(as against all source collections in the index definition before.)

We acknowledge that this is a bit late in the cycle, but the change should be 

of lesser risk and could a desirable change for making the multi-collection index

really multi-tenant)

Tagging Evgeny Makarenko/Keshav Murthy for inputs.