Refactor ReactiveCRUD

[mathieuancelin]

Avoid Future[Unit]

[gmarizy]

The following are just details without consequences, but speaking about refactoring ReactiveCRUD:

• Why using sel: (View, Query) instead of something like view: View, query: Query in find, findstream and other methods ? I don't see the point using a tuple there, and sel._2 is less readable than a properly named parameter.
• implicit def ctx: ExecutionContext// = implicitly[ExecutionContext] why is the default value commented ?
• In the same idea, val format may have a implicitly[Format[T]] similar to implicit writer and reader in CouchbaseCrudSource in ReactiveCouchbase-play.