Refactor RPC Client creation

[tzdybal]

Interaction points between rollmint and cosmos-sdk are limited to Node and RPC Client.

• Node currently needs to satisfy Service interface from tendermint (actually, only Start method matters).
• RPC Client needs to implement tendermint/rpc/client.Client interface

Currently we're creating RPC client explicitly in cosmos-sdk:

https://github.com/celestiaorg/cosmos-sdk-rollmint/blob/81666de7b66b8bc1af8b5d532be853c4e84bc137/server/start.go#L359-L360

This code is "owned"/changed by us anyways, so we can easily change it.
Proposed change is to create minimalistic Node interface:

type Node interface {
    Start() error
    GetClient() rpc.Client
}

This way, each concrete implementation of Node interface will be able to provide concrete implementation of RPC Client.
Note: trying to create single rpc.Client implementation working with multiple node types leads to very bad "abstraction" of Node.

Concrete tasks:

• Refactor Node interface #662
• Use existing RPC Client implementation only by FullNode #663
• Create RPC Client for LightNode #664
• Use Node.GetClient to create RPC client cosmos-sdk-old#15
• Remove direct use of EventBus in rpc/json/service.Subscribe #682
• Add IsRunning and Stop methods to Node interface #705