Revision 665455

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.