Rename packages with `_` in their names

[bsbodden]

Remove underscores in the packages/folders below (suggestions added)
src/main/java/com/redislabs/redisgraph/graph_entities ==> src/main/java/com/redislabs/redisgraph/entities
src/main/java/com/redislabs/redisgraph/impl/graph_cache ==> src/main/java/com/redislabs/redisgraph/impl/cache