Make SDK Rootchain module

[colin-axner]

Summary

Port solidity rootchain contract into SDK module

Problem

To get plasma on cosmos I think it makes a lot of sense to turn the rootchain contract into a module that zone can simply add on if desired. No need to wait for ethermint, this would allow for plasma on non ethermint zones.

Proposal

Sidechain requires few changes, mostly would just need to rework, key/signature processing and connection to other tm nodes as opposed to eth nodes