go-replay middleware/plugin for a/b regression testing

[bartekn]

When testing new Horizon release candidate for regression bugs we use:

• horizon-cmp to replay access log requests and check diffs between base and test versions,
• go-replay to replay production cluster requests live as they come, however it doesn't check diffs between base and test clusters (it tests performance though).

We can write a go-replay middleware/plugin to add horizon-cmp functionality to go-replay. Middlewares have access to original response and replayed response. Some code from horizon-cmp can be useful, ex. excluding expected changes.