test fetcher: add more stats

Change-Id: Iad880544174c812a49640939b8fb2922da738b28
schema: some FileReader cleanups: use blob.Ref as map key, simplify zeroReader

Change-Id: I37ec27bfe9673e7d5566216269d94d109215ac99
schema: reduce FileReader blob reads 32-85x by caching the last blob read

Little reads into big blobs kept re-reading the same blob.

Change-Id: I098c9d9a9443dacc93f60f96fff1edd421ced198