Rename packages with `_` in their names

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` 