StreamingFetcher is now just Fetcher, and its FetchStreaming is now
just Fetch.
SeekFetcher is gone. Blobs are max 16 MB anyway, so we can slurp to
memory when needed. The main thing that cared about SeekFetcher
was the GET handler, ServeBlobref, because http.ServeContent needed
one for range requests. That's rewritten in an earlier commit, using
the FakeSeeker from another earlier commit.
Lot of code got simpler as a result.
Change-Id: Ib819413e48a8f9b8d97f596d0fbf771dab211f11
Brad Fitzpatrick