Bug #2865: Reduce Keep server memory use
Review @ 70dd308
Eliminated ioutil.ReadAll, which was naively allocating buffers 2x as large as necessary.
- Rewrote main.PutBlockHandler to allocate a buffer of exactly req.ContentLength bytes, and fill it with io.ReadFull.
- UnixVolume.Read now uses ioutil.ReadFile to read the contents of a block file (which it should have been doing in the first place)