Clean up RPC API endpoints

[karlfloersch]

Is your feature request related to a problem? Please describe.
Currently the RPC requests are handled very poorly with if and else if

Offending files:

• https://github.com/plasma-group/plasma-chain-operator/blob/master/src/server.js
• https://github.com/plasma-group/plasma-chain-operator/blob/master/src/state-manager/app.js
• https://github.com/plasma-group/plasma-chain-operator/blob/master/src/block-manager/app.js

Describe the solution you'd like
Change it to look like b-coin