c++: performance comparison of old and new API

[Alan Conway <ac...@redhat.com>]

I've extended & added to the new API tests to get some performance data. We now 
have qpid_cpp_benchmark, a python script that orchestrates instances of C++ 
clients qpid_send and qpid_receive to give something like perftest. I think 
separate send & receive tests will be more flexible than a perftest-like monolith.

Attached are some results. The main conclusions I draw:
- there's a serious (50%) reduction in throughput. I'm not certain if this is 
the API itself or a mistake in the test.
- latency looks better with the new API at a rate of 30k. The latency numbers 
for higher rates don't mean much since the new api is topping out at 30k.
- there's surprisingly little improvement in going to unreliable delivery in the 
new API, vs. the default reliable delivery. With a different build (-O rather 
than -O3) reliable was actually faster. This makes no sense to me.

If anyone has thoughts on how the tests could be improved or theories to explain 
the results I'm very interested.