additional changes as follows:
* LTRFeatureLoggerTransformerFactory:
** feature values cache name configurable (instead of hard-coded value that needs to match solrconfig.xml configuration)
** javadocs (example and parameters)
* CSV FeatureLogger:
** removed delimiter and separator assumptions in tests
** changed delimiter and separator (from "key:val;key:val" to "key=val,key=val")
** configurable (key value) delimiter and (features) separator
* JSON FeatureLogger:
** defer support for this (removing MapFeatureLogger class)
* adds 'training libraries' to (Linear|MultipleAdditiveTrees)Model javadocs
(Diego Ceccarelli, Michael Nilsson, Christine Poerschke)
* removed code triplication in ManagedModelStore
* LTRScoringQuery.java tweaks
* FeatureLogger.makeFeatureVector(...) can now safely be called repeatedly (though that doesn't happen at present)
* make Feature.FeatureWeight.extractTerms a no-op; (OriginalScore|SolrFeature)Weight now implement extractTerms
* LTRThreadModule javadocs and README.md tweaks
* add TestFieldValueFeature.testBooleanValue test; replace "T"/"F" magic string use in FieldValueFeature
* add TestOriginalScoreScorer test; add OriginalScoreScorer.freq() method
* in TestMultipleAdditiveTreesModel revive dead explain test
