Refactor gateways

[VaibhavPage]

**Is your feature request related to a problem? **
Currently the framework offers different flavors of gateway like core, http and grpc that an user can implement. But this has lead to bloated implementation and in case of http and grpc gateways, framework needs to add middleware components that can talk over different communication protocols.

Describe the solution you'd like
Communication between framework and user's gateway code should happen only over gRPC.
This will standardize how gateways are implemented and it will make code easy to maintain.
Also it will be easier to manage state of the event sources.

[VaibhavPage]

@magaldima @spk83 @dtaniwaki thoughts?

[dtaniwaki]

@VaibhavPage I agree with you. That reduces lots of redundant code and simplifies gateways. 👍