watcher cancel running watches

[fkowal]

ConsulWatcher.stopWatchingEndpoint(String endpoint) limits the possibility to use the watcher concurrently.

When 2 threads start watches for the same endpoint, and one decides to cancel the "subscription" by running .stopWatchingEndpoint(String Endpoint) it results in canceling both subscriptions.

Or it introduces a race condition that results in leaving both subscriptions running.