Thrift: Move TStringBuffer functionality into TMemoryBuffer.

Thrift: Move TStringBuffer functionality into TMemoryBuffer.

Summary:
TMemoryBuffer already has the necessary plubming to work with C++ strings.
This revision implements that functionality with a few wrapper methods.
Removed TStringBuffer as it should no longer be required (and it is tricky
to safely inherit from a class that has a non-virtual destructor).
Also refactored the TMemoryBuffer constructors a bit.

Reviewed By: aditya, yunfang

Test Plan:
test/TMemoryBufferTest.cpp

Revert Plan: ok

git-svn-id: https://svn.apache.org/repos/asf/incubator/thrift/trunk@665213 13f79535-47bb-0310-9956-ffa450edef68