blobserver/handler: rename conn to rw, per more recent conventions

Change-Id: Ibd28bc66f9d1f6c9dac776ce78a9da9baddba7c4
Backwards-compatible protocol + handler change on boundary of enumerating blobs.

The handler was adding 1 to things to see if it was at the end, but that was causing
HTTP requests to Amazon like: limit 1000, limit 1, limit 1000, limit 1, etc.

This makes it twice as fast.

Change-Id: Ibb7e3f6ae7229a21c87817c7438324d36e7b491a