[Postponed] Replace IDocumentClientEx with proper SDK interface -- when it comes online

[kadukf]

Extend DocDB SDK so that IXyzEx interfaces + their implementations are not needed.

[ealsur]

The current IDocumentClient does not yet expose CreateDocumentChangeFeedQuery and ReadPartitionKeyRangeFeedAsync.

We need those two methods in order to replace the actual interfaces completely.

[mkolt]

Do we need this? SDK Ixxx interfaces would probably always be behind. IDocumentClientEx + others are not bad.

[mkolt]

SDK changes are not going to be in nearest future. Will have to postpone this until then.

[mkolt]

We should close this.

IChangeFeedDocumentClient is self-consistent interface that outlines what is needed to CFP and has worked well. IDocumentClient with its 200+ methods it too much.