Connectivity issues in IPv4-only and IPv6-only configurations

[bavshin-f5]

Our current connection logic always takes the first entry from a combined list of A and AAAA DNS query results in arbitrary order. On a system that supports a single address family, either by local configuration or due to a routing issues, this may result in intermittent connection errors.

To address this, we have to make several connection attempts using both protocols, preferably with RFC6555 fast fallback logic to reduce the timeout.

The workaround is to pass ipv4=off/ipv6=off to the resolver directive.