An abstract class implementing Sarathi's discovery API.
###getDiscoveredInstances Return the array containing instance information of discovered instances. The format of these instance objects is:
[{
address: "", // ip address OR domain
port: "", // port for use
url: "" // form a url with the address and port and supported protocol, the protocol can be asked by the strategy, or decided, but the url should have it.
}]###discoveryDone(successHandler: Function, failureHandler: Function)
Trigger the callback when discovery is done and instances are ready to be pulled. Only when the success callback is invoked, any calls to the APIs will be triggered. If promise is already resolved, it should trigger the callback immediately. Behavior of Sarathi, on invocation of error handler depends on availability of nodes to use from previous load.
###discoverInstances Method that indicates that sarathi is configured and ready to start discovery of services.