Fix CACHE_WRITE_STRATEGY

This fixes https://github.com/graphite-project/carbon/issues/234 by setting the global state object later in the load process. 

The MetricCache singleton was being loaded when `carbon.instrumentation` was loaded, which was before the configuration was parsed. The result was only default value were visible during MetricsCache init.

I added a few tests to try and cover/discover the difference in cache write ordering, but more work can be done there as well. My own testing shows that this works as expected, but I'd appreciate more :eyes:.