Thrift: TDenseProtocol.

Thrift: TDenseProtocol.

Summary:
- Made some stuff in TBinaryProtocol protected instead of private.
- Added a preliminary version of TDenseProtocol.  This is still
  super highly experimental and gross, and I wrote a super scary
  comment to explain that to anyone foolish enough to try to use
  this in its current state.

Reviewed By: mcslee

Test Plan: test/DenseProtoTest.cpp

Revert Plan: ok

Memcache Impact:
Save memory if/when people use it.

git-svn-id: https://svn.apache.org/repos/asf/incubator/thrift/trunk@665247 13f79535-47bb-0310-9956-ffa450edef68