[all] Watcher module

[LayneHaber]

This is a tracker issue for the creation of the watcher module, spec.

Relevant issues:

• [x] create types for initialization, interfaces #1034
• [x] implement ChainListener class #1035
• [x] implement Watcher contract calls, disable / enable #1036
• [x] implement Watcher dispute initiation #1037
• [ ] add watcher to node/client #1038
• [x] run watcher tests in CI https://github.com/connext/indra/issues/1220