General Performance Improvements for Throughput

I came up with a couple of small tweaks:
https://github.com/datastax/python-driver/pull/375
The changes are minor, and only amount to 1-2% improvements (depends on workload), but worth making.
The PR also contains some unrelated changes removing dead code and refactoring tests.

There is actual cython optimization work going on here: https://github.com/datastax/python-driver/compare/master...markflorisson:cython

Looks good.