(1) Netty's ByteBuf cannot support data > 2gb.  So to transfer data from a
ChunkedByteBuffer over the network, we use a custom version of
FileRegion which is backed by the ChunkedByteBuffer.

(2) On the receiving end, we need to expose all the data in a
FileSegmentManagedBuffer as a ChunkedByteBuffer.  We do that by memory
mapping the entire file in chunks.

Added unit tests.  Also tested on a cluster with remote cache reads >
2gb (in memory and on disk).