add compression to diskpacked

Initial commit for implementing compression in diskpacked
- with fixed ratio of 0.75 (no compressed storage if the saving is
less than 25%), zlib compression method, default level.

As blob.fetcherToSeekerWrapper.Fetch erroneously asserts that FetchStreaming
returns a ReadSeekCloser everytime, it had to be changed.
This is addressed with a readerutil.MakeReadSeekCloser
(pkg/readerutil/memfile.go) which slurps the reader into memory, or spoils
it to temp file if too big.

This utility now is also used in pkg/blobserver/diskpacked.ReceiveBlobs.

Change-Id: I5ee59c2ac325768f0e29a4dfee9a437f6f1384bd