* feat: promisify all api methods that accept callbacks
This is a stop-gap until the full async/await migration can be
completed. It means we can refactor tests of other modules that
depend on this module without having to mix async flow control
strategies.
N.b. some methods that were previously callable without callbacks
(e.g. `node.start()`, `node.stop()`, etc) now require callbacks
otherwise a promise is returned which, if rejected, can cause
`unhandledPromiseRejection` events and lead to memory leaks.
* docs: add a global note to the api about promisify
* fix: update the logic for unsubscribe
* test(fix): correct pubsub unsubscribe usage for api change
* test(fix): update content routing tests for latest delegate version
* docs: update chat example and add info to its readme
* docs: update echo example
* docs: update libp2p in browser example
* docs: update pubsub example
* docs: update peer and content routing examples
* docs: update discovery mechanisms example
* docs: update encrypted comms example
* docs: update protocol and stream muxing example
* feat: add config validation
* test: update CI configs, use only node 8
