In DNS service watcher, re-resolve every $TTL

[tomfitzhenry]

The DNS service watcher will re-resolve every check_interval seconds. Rather than having to set check_interval, and possibly setting it too high/low, it would be nice if it re-resolved every $TTL (whatever the returned TTL is).

[tomfitzhenry]

I can think of three approaches for this:

1. For each server in @discovery['servers'], start a new watcher thread that does DNS resolution for that server, and sleeps for $TTL.
2. Start a single watcher thread that does DNS resolution for all servers, and sleeps for the minimum across all TTLs, before re-resolving them all.
3. Start a single watcher thread that does DNS resolution for all servers, and sleeps for the smallest TTL, then re-resolving the expired DNS record, then sleeps for whatever the smallest TTL is (maybe the same server, maybe another one),

The problem with (1) is that for N servers, there are N threads. This might be a problem for users if N is large.

The problem with (2) is that if N is large, we'll be doing a lot of resolving.

The problem with (3) is the code is trickier, but I'll give it a go.