Removing dependency on cassandra's DebuggableThreadPoolExecutor.
While running thw unit tests for priam 1.2, it failed to try to execute a c* 1.2 Tracer
function. Looking into it, DebuggableThreadPoolExecutor now tries to do Traceing on most calls,
if it can. Since this is running inside of priam, we really don't need to do any c* Tracing
(it'll fail horribly). So I created a new TPE and use that. The existing CustomizedThreadPoolExecutor
had some specialized behavior built into it's submit method that I felt uncomfortable
changing at this time. I renamed that class, nonetheless, to something more specific.