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