9068: Move buffer allocation from volumes to GetBlockHandler.

9068: Move buffer allocation from volumes to GetBlockHandler.

This makes the Volume interface more idiomatic: Get() accepts a buffer
to read into, and returns a number of bytes read, much like the Read()
method of an io.Reader.

It also makes it possible for GetBlockHandler to notice, while waiting
for a buffer, that the client has disconnected: In this case, it
releases the network socket and never asks any volumes to do any work.