RFR: HTTP query for new rollup types.

1194b3c and 1194b3c set things up in other classes. The only questionable changes are the `hasData()` alterations. I think keying on `count` is more correct than `sampleCount`.

03e539c fixes `AstyanaxReader` so that it reads from the correct CF during retrieval.  This is done by tracking rollup type in the metadata cache.

The bulk of work is in 02c9fc3.  This centers around proper JSON serialization of the new rollup types.

Tests are in 9a715ab.