dotNAV: Add option to return subdomains from a name

[aguycalled]

@sakdeniz requested to add the option to the RPC command resolvename to return a name's subdomain.

• [x] Required changes in navcoin-core
• [x] Required changes in electrumx
• [x] navcoin/navcoin-js#7
• [x] navcoin/infrastructure#1
• [x] Testing and confirmation from @sakdeniz
• [ ] Update mainnet infrastructure

[aguycalled]

@sakdeniz the changes have been implemented, could you confirm they work?

[sakdeniz]

I will test it soon.

[sakdeniz]

Tested, its works as expected.