Thrift: New methods for C++ transports.

Thrift: New methods for C++ transports.

Summary:
To support the upcoming variable-length encoding of integers in
TDenseProtocol, augment Thrift transports with two new methods:
borrow and consume.  Borrow copies some data out of the transport
without consuming it, and consume... consumes it.

Reviewed By: mcslee

Test Plan:
Thrift compiles.
Should check in variable-length integer code for TDenseProtocol
right after this (git ruulz).

Revert Plan: revert stuff that uses these methods also.

git-svn-id: https://svn.apache.org/repos/asf/incubator/thrift/trunk@665251 13f79535-47bb-0310-9956-ffa450edef68