ResultCache impl for extensions of DataConnector

[Rod Widdowson]

> I was planning to punt it to V4 but we can talk about it.

That was my assumption too, but it would be nice to know what people are using/want to use.

And there is an odd sorta-philosophical question as well given that 3.4 is the last release pre V4 (when APIs will change):  Is it
worth changing anything in 3.4?  

	Customer uses impl now -> it might break at any time
	Customer uses Api in 3.4 -> it might break in V4

With the only difference being that we are more likely to take care about api breakages (and avoid them as much as possible).

R