Implement an Informer in python-client

https://github.com/kubernetes/client-go has an Informer implementation. Internally it leverages a watcher of some collection of resources, continually streams changes (add/modify/delete events), reflects the resources into a downstream store (cache), handles connectivity drops, and periodically does a full resync. This all happens on some background thread (goroutine). A client of the informer is free to iterate over that stored cache without concern for how it's populated, and immediately get (possibly outdated) state.

Applications using https://github.com/kubernetes-client/python that want a local store of resources reflecting some in-cluster state need to concern themselves with those lower-level details. There's a lot of room for error.

On 2019-06-25, go#28 added a simple informer implementation to the openapi-generated client for Golang. It defines a `Cache` struct, with both a list of all objects and event handler callbacks that a consumer could register.

https://github.com/kubernetes-client/python should contain a similar implementation.

People have been talking about this a bit in various places.

* kubernetes-client/go#28 is a PR adding a simple informer implementation to the openapi-generated client for Golang (in contrast to the more mature https://github.com/kubernetes/client-go).
* kubernetes-client/python#124 is about reconnection, no cache
* kubernetes-client/python#334 is about a sample controller and briefly discusses the lack of an informer
* bazelbuild/rules_k8s#55 doesn't implement a cache; it's an example of a watch
* https://borismattijssen.github.io/articles/kubernetes-informers-controllers-reflectors-stores has a nice high-level description of the whole flow from the perspective of consuming the client-go implementation
* https://kubernetes.slack.com/archives/C0EG7JC6T/p1520698801000158 has @marcellodesales and @liggitt discussing options in 2018-March
* https://kubernetes.slack.com/archives/C09NXKJKA/p1504044752000260 is confusion from @dimberman about the lack of a python informer in 2017-August