Probe: provide API to programmatically execute queries and handle results

Currently, probe gets its results as a plain text, read from a datagram socket. An API would allow clients to gets responses aggregated by cluster member, in hashmap format. A client should be able to register and get notified whenever a response was received, and also retrieve the entire response set after a timeout.
Such an API makes it simpler for (e.g.) management applications to ping the cluster members and get information about them. Without the API, a client has to parse the returned text and arrange it into keys and values.