Remove unnecessary printf debugging
Only create one logging thread per logging buffer

This was probably causing the occasional duplicate log message
The client test application now has a speed test option
Add a client for performance testing the non-Lwt code

This has exposed a serious performance problem: against the same (Lwt)
server, the non-Lwt client manages 14 RPCs/sec while the Lwt version
manages around 980 RPCs/sec.