Inner class FieldCacheSources should be refactored into their own classes

While working on SOLR-1586 I noticed that outside of class level access (or package level), you can't really reference FieldCacheSources that are defined inside of their FieldType constituents (e.g., in the case of StrFieldSource as defined in StrField). What's more troubling is that the FieldType/FieldCacheSources are defined in an inconsistent fashion: some are done as inner classes, e.g., StrFieldSource and SortableFloatFieldSource, while others are defined as individual classes (e.g., FloatFIeldSource). This patch will make them all consistent and define each FieldCacheSource as an outside class, present in o.a.solr.search.function.