eth: unify the eth receiver name (s, eth) -> e

[georgehao]

Purpose

Currently, eth.Ethereum have two named receiver s and eth. This don't follow golang code style.

s receiver: https://github.com/ethereum/go-ethereum/blob/master/eth/backend.go#L309 eth receiver: https://github.com/ethereum/go-ethereum/blob/master/eth/state_accessor.go#L177

So, this pr unify the receiver name

[holiman]

What is the logic for calling Ethereum s ?

[georgehao]

What is the logic for calling Ethereum s ?

I noticed that most of the receivers were originally called s. Of course, they can be unified as eth for more clarity.

But, backend.go is in the eth package. if the receiver is also call eth, maybe confused. eg:

eth.Etherbase()
eth.MakeProtocols()

developer can't recognize whether the function is a package method or struct method. And this will cause error invoke. Maybe receiver name callse is more suitable

[lightclient]

Maybe was s for "service" ?