[SPARK-6229] Add SASL encryption to network library.

There are two main parts of this change:

- Extending the bootstrap mechanism in the network library to add a server-side
  bootstrap (which works a little bit differently than the client-side bootstrap), and
  to allow the  bootstraps to modify the underlying channel.

- Use SASL to encrypt data going through the RPC channel.

The second item requires some non-optimal code to be able to work around the
fact that the outbound path in netty is not thread-safe, and ordering is very important
when encryption is in the picture.

A lot of the changes outside the network/common library are just to adjust to the
changed API for initializing the RPC server.

Author: Marcelo Vanzin <vanzin@cloudera.com>

Closes #5377 from vanzin/SPARK-6229 and squashes the following commits:

ff01966 [Marcelo Vanzin] Use fancy new size config style.
be53f32 [Marcelo Vanzin] Merge branch 'master' into SPARK-6229
47d4aff [Marcelo Vanzin] Merge branch 'master' into SPARK-6229
7a2a805 [Marcelo Vanzin] Clean up some unneeded changes.
2f92237 [Marcelo Vanzin] Add comment.
67bb0c6 [Marcelo Vanzin] Revert "Avoid exposing ByteArrayWritableChannel outside of test code."
065f684 [Marcelo Vanzin] Add test to verify chunking.
3d1695d [Marcelo Vanzin] Minor cleanups.
73cff0e [Marcelo Vanzin] Skip bytes in decode path too.
318ad23 [Marcelo Vanzin] Avoid exposing ByteArrayWritableChannel outside of test code.
346f829 [Marcelo Vanzin] Avoid trip through channel selector by not reporting 0 bytes written.
a4a5938 [Marcelo Vanzin] Review feedback.
4797519 [Marcelo Vanzin] Remove unused import.
9908ada [Marcelo Vanzin] Fix test, SASL backend disposal.
7fe1489 [Marcelo Vanzin] Add a test that makes sure encryption is actually enabled.
adb6f9d [Marcelo Vanzin] Review feedback.
cf2a605 [Marcelo Vanzin] Clean up some code.
8584323 [Marcelo Vanzin] Fix a comment.
e98bc55 [Marcelo Vanzin] Add option to only allow encrypted connections to the server.
dad42fc [Marcelo Vanzin] Make encryption thread-safe, less memory-intensive.
b00999a [Marcelo Vanzin] Consolidate ByteArrayWritableChannel, fix SASL code to match master changes.
b923cae [Marcelo Vanzin] Make SASL encryption handler thread-safe, handle FileRegion messages.
39539a7 [Marcelo Vanzin] Add config option to enable SASL encryption.
351a86f [Marcelo Vanzin] Add SASL encryption to network library.
fbe6ccb [Marcelo Vanzin] Add TransportServerBootstrap, make SASL code use it.