Discovery: send multiple discovery requests

Define a num_requests (default: 1) attribute (we used to have this!) that governs how many times a discovery is executed until the timeout is reached, or a coord is found.

This may be useful when multiple members are started at the same time and/or the discovery does not return the correct members the first time around. Especially needed for DNS_PING.

The number of requests should be spaced out evenly across a range [0 .. timeout-(timeout/num_requests)]

Example: timeout=12000, num_requests=3;

• Range: [0 .. 8000] (12-4)
• First req sent at time 0
• Second: time 4000
• Third: 8000
  Interval: timeout - (timeout/num_reqs) / num_reqs-1