Refactor frontend request handler

[v0y4g3r]

In #456 we've moved SQL parsing stuff to frontend, but some work remains unfinished. As mentioned in this comment, both standalone and distributede mode, frontend communicates with datanode thru gRPC invocation, which causes unncessary serialization overhead. Thus we need to introduce an intermediate representation and a trait for frontend request handling. For example, in standalone mode, this trait is implemented by method call, while in distributed mode, this trait is implemented by gRPC invocation.

In issue #597, @waynexia is working on the substrait intermediate representation and some progress's been made. Thus we can proceed refactoring the frontend.