Better portainer edge agent support for "round-robin" dns.

[gorucci]

Is your feature request related to a problem? Please describe

I've used portainer edge agents for about a year and recently remade my network. During this period, portainer was accessible via two IPs. So I've set the DNS A record for portainer to 2 IPs, thus bringing a "zero-downtime experience".

After I'd finished network migration, portainer was accessible only via one (new) IP. And I forgot to remove the second IP from the A record🤦‍♂️. Since then, the connection to the edge agent has been very unstable, but I couldn't understand why... After I've revised A records and removed the old IP, all went back to normal.

Describe the solution you'd like

If the portainer edge agent can't connect to the one IP (resolved from DNS), it should try to connect to another (just as browsers and other modern applications do). It seems that it just picks an IP randomly now (and tries to connect indefinitely).

Describe alternatives you've considered

Welp, the current implementation is okayish.

Additional context

No response