OSS port of enterprise PR 1822
Includes the necessary changes to the `proxycfg` and `xds` packages to enable
Consul servers to configure arbitrary proxies using catalog data.
Broadly, `proxycfg.Manager` now has public methods for registering,
deregistering, and listing registered proxies — the existing local agent
state-sync behavior has been moved into a separate component that makes use of
these methods.
When an xDS session is started for a proxy service in the catalog, a goroutine
will be spawned to watch the service in the server's state store and
re-register it with the `proxycfg.Manager` whenever it is updated (and clean
it up when the client goes away).
* Proxy Config Manager
This component watches for local state changes on the agent and ensures that each service registered locally with Kind == connect-proxy has it's state being actively populated in the cache.
This serves two purposes:
1. For the built-in proxy, it ensures that the state needed to accept connections is available in RAM shortly after registration and likely before the proxy actually starts accepting traffic.
2. For (future - next PR) xDS server and other possible future proxies that require _push_ based config discovery, this provides a mechanism to subscribe and be notified about updates to a proxy instance's config including upstream service discovery results.
* Address review comments
* Better comments; Better delivery of latest snapshot for slow watchers; Embed Config
* Comment typos
* Add upstream Stringer for funsies
Dan Upton