WIP: StringMetricsPersistenceOptimizer will drop string metrics if so configured

The Config value is in CoreConfig.java. 

I made the change here instead of AstyanaxWriter so that we know exactly where the code has been changed and also we need to check the config value only once. 

Review to see 
* Enough tests have been added
* Change is cleaner than https://github.com/gdusbabek/blueflood/commit/326906937ee5d782fb0296cf7dab1fa03f0f3929
perhaps this should be a static field on the class