Thrift: Make borrow (almost) always succeed for TBufferedTransport.

Thrift: Make borrow (almost) always succeed for TBufferedTransport.

Chad Walters is writing a JSON protocol for Thrift, but he wants
borrow to always succeed.  That would be a pain to implement,
but here is a first step: borrow will almost always work with
TBufferedTransport.

Reviewed by: mcslee

Test Plan: Ran the DenseProtocol test and Zlib test, but more needs to be done.

Other Notes:
Also reviewed by Chad Walters, and maybe Ben Maurer.

git-svn-id: https://svn.apache.org/repos/asf/incubator/thrift/trunk@665455 13f79535-47bb-0310-9956-ffa450edef68