WIP: StringMetricsPersistenceOptimizer will drop string metrics if so configured

I'd ideally like to close this PR out in favor of #388
#388 gets rid of all the hackyness here and simplifies the optimizer code

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