Thrift: Update the interface for TTransport's "borrow" method.

Thrift: Update the interface for TTransport's "borrow" method.

Summary:
I don't know what I was thinking when I first wrote this.
It makes sense that the transport might not want to allocate its own memory,
so the protocol is expected to provide a buffer for the data.
However, if the transport already has the data buffered,
there is no need to memcpy it; it can just return a pointer into its buffer.
The new interface still requires the protocol to provide a buffer,
but allows the transport to return a pointer to an interal buffer.
In addition, I made len a pass-by-pointer parameter so that
the transport can return more than the requested data if it has it
available in its buffers.

Reviewed By: mcslee

Test Plan: Ran the DenseProtocol test and the Zlib test.

Revert Plan: ok

Other Notes:
Also got this reviewed by Chad Walters from Powerset.
Ben Maurer suggested making len a reference parameter.

git-svn-id: https://svn.apache.org/repos/asf/incubator/thrift/trunk@665454 13f79535-47bb-0310-9956-ffa450edef68