Spike: Implement CometMock prototype in Go

[p-offtermatt]

Problem

A decision was made to use Tendermock for our e2e tests, but to reimplement it in Go.
I propose to also change the name to CometMock, to make sure there are no concerns regarding the Tendermint brand.
For this issue, a very small prototype implementation is planned to allow better splitting the work on doing the
full reimplementation.

The goal is to allow a simple stress test: connecting to N validators and processing M token transfers.
This will let us know if the implementation in Go still has performance bottlenecks.

Desired features thus are:

• Connect to multiple applications via ABCI
• Provide minimal RPC hooks to allow broadcasting token transfers

Closing criteria

The desired features have been implemented in a prototype implementation in Go.

[p-offtermatt]

CometMock implementation, for now, lives in its own repo with a PR for the content of this issue informalsystems/CometMock#1