[FEATURE] Add DNS resolver to acquire seed peers

[alexeykiselev]

Аbstract
A gowaves node knows IP addresses of seed nodes (initial nodes to connect to) from the built-in configuration. Also the initial nodes could be provided by the command line parameter -peers. It seems better to replace or the built-in seed peers with the DNS record lookup.

Specification
Built-in configuration should support domain names along with seed nodes IP addresses. Those domain names should be resolved to underlying IP addresses and those addresses should be treated as usual seed nodes IP addresses.
The argument of -peers command line parameter should be parsed the same way as built-in configuration.

Backwards Compatibility
Configuration and -peers command line parameter should support IP addresses passing for backward compatibility.

Examples and Implementation
Bitcoin operates the same way.
https://github.com/btcsuite/btcd/blob/535f25593d47297f2c7f27fac7725c3b9b05727d/connmgr/seed.go#L33