Rework Sotw Cache interface

[valerian-roche]

Datadog fork PR matching envoyproxy#584

It has been raised that the addition of StreamState within the Cache interface is breaking the separation of concern between the server side managing the xds protocol itself and the cache which is only in charge of returning responses based on watches
PR envoyproxy#577 has been opened proposing to fully remove any client state from the CreateWatch interface

This PR is an alternate proposal based on a few concerns/aspects:

• one of the argument on removing it is that the sotw protocol is stateless. It actually is not, and is explicitly visible in this PR: the lastResponse latched value as well as the version hack when the resource list is changed very well show that it is stateful
• in cases outside of lds and cds, it is clearly stated that the control-plane is not expected to send back all resources when a single one change. This is highly critical in the case of ads with eds, as there is a likelihood of both having a lot of resources and a high churn. The implementation sending everything each time is technically tolerated, but is pushing on the data-plane work that could be done at low cost on the control-plane
• the current support of both delta and sotw, for both linear and simple cache means that every change has to be done 4 times, with four very different implementations, each one supporting only a subset of use cases (e.g. linear does not support cds/lds as it doesn't handle wildcard in this case, and sotw does not work properly as resource list is improperly maintained). I strongly believed that both implementation should converge. The only remaining differences should be in the messages themselves, abstracted in the server part. Having a common interface for sotw and delta is a first step for this. It would also allow in the future to make snapshot far more efficient by supporting a linear version per snapshot

In this context, this PR is changing:

• creates a ClientState within the cache package, driven by the data needed from the cache perspective to be able to answer a client request
• simplifies streamState
  • makes it compatible with the new ClientState interface
  • removes the IsFirst notion, as the protocol states it should come from the nonce in the request
  • removes the duplicate/parallel knownResourceNames and use the delta one instead. It will also properly carry the version in a future PR

[atollena]

I guess you can close this PR.