Extend ska-pact-tango with async events

We can use PACT to simplify integration testing.

Tests of APIs can be done exclusively using PACT, achieveing a complete decoupling of the consumer team from the provider team. This inclreases flexibility and reliability of the testing process.

A second benefit is that contracts are the materialization of the API, and PACT tests can be used to determine if the consumer/provider are compliant wrt the API specification. 

• I can specify a contract for a consumer that registers a callback against an event
• the contract allows me to specify conditions/properties of the payload of the event
• PACT creates and injects a mock of the provider so that I can test the consumer
• PACT supports verification of the contract against the provider

A study needs to be included on what it would mean to extend the contract specification so that N providers and a particular sequence of events can be specified, as opposed to having a single provider and only one event. The rationale here is that most of the challenging problems appear when dealing with different ordering of the events, with possible race conditions, with different timings and delays of events. It would be good to be able to specify these conditions in the contract.