`proxycfg.State.Watch` not protected against multiple invocations

https://github.com/hashicorp/consul/blob/20eefeea117dcd23e30dab942acd2cf007024d9e/agent/proxycfg/state.go#L93-L109

This overwrites the current ctx and cancel function, executes the `run` method in a go routine and returns a chan.

The problem is that if Watch is called multiple times we could:

1. Lose the current ctx/cancel and potentially leak some go routines
2. Completely break everything related to this `proxycfg.State`. They will be sharing some internal chans so it would be possible for neither instance of the go routine running to get a complete view of the proxy configuration and never think its configuration is valid.

At this point this is all purely theoretical but we may want to prevent this in the future.