This change seems large, but most of it is just replacing `byte[]`
with `ByteBuffer` and `new byte[]` with `ByteBuffer.allocate()`,
since it changes the network library's API.

The following are parts of the code that actually have meaningful
changes:

- The Message implementations were changed to inherit from a new
  AbstractMessage that can optionally hold a reference to a body
  (in the form of a ManagedBuffer); this is similar to how
  ResponseWithBody worked before, except now it's not restricted
  to just responses.

- The TransportFrameDecoder was pretty much rewritten to avoid
  copies as much as possible; it doesn't rely on CompositeByteBuf
  to accumulate incoming data anymore, since CompositeByteBuf
  has issues when slices are retained. The code now is able to
  create frames without having to resort to copying bytes except
  for a few bytes (containing the frame length) in very rare cases.

- Some minor changes in the SASL layer to convert things back to
  `byte[]` since the JDK SASL API operates on those.

Author: Marcelo Vanzin <vanzin@cloudera.com>

Closes #9987 from vanzin/SPARK-12007.