[SPARK-6578] [core] Fix thread-safety issue in outbound path of network library.

While the inbound path of a netty pipeline is thread-safe, the outbound
path is not. That means that multiple threads can compete to write messages
to the next stage of the pipeline.

The network library sometimes breaks a single RPC message into multiple
buffers internally to avoid copying data (see MessageEncoder). This can
result in the following scenario (where "FxBy" means "frame x, buffer y"):

               T1         F1B1            F1B2
                            \               \
                             \               \
               socket        F1B1   F2B1    F1B2  F2B2
                                     /             /
                                    /             /
               T2                  F2B1         F2B2

And the frames now cannot be rebuilt on the receiving side because the
different messages have been mixed up on the wire.

The fix wraps these multi-buffer messages into a `FileRegion` object
so that these messages are written "atomically" to the next pipeline handler.

Author: Marcelo Vanzin <vanzin@cloudera.com>

Closes #5234 from vanzin/SPARK-6578 and squashes the following commits:

16b2d70 [Marcelo Vanzin] Forgot to update a type.
c9c2e4e [Marcelo Vanzin] Review comments: simplify some code.
9c888ac [Marcelo Vanzin] Small style nits.
8474bab [Marcelo Vanzin] Fix multiple calls to MessageWithHeader.transferTo().
e26509f [Marcelo Vanzin] Merge branch 'master' into SPARK-6578
c503f6c [Marcelo Vanzin] Implement a custom FileRegion instead of using locks.
84aa7ce [Marcelo Vanzin] Rename handler to the correct name.
432f3bd [Marcelo Vanzin] Remove unneeded method.
8d70e60 [Marcelo Vanzin] Fix thread-safety issue in outbound path of network library.