node: accept --no-dns configuration option to disable NS and RS

[pinheadmz]

Closes #528

Summary (also included in CHANGELOG):

• FullNode and SPVNode accept configuration parameter --no-dns (or no-dns: true in
  hsd.conf) which launches the node without either DNS server (the root authoritative
  server and the recursive resolver). This avoids some port collisions with other HNS resolvers
  like hnsd running locally, and generally separates and reduces security concerns around
  running unneeded servers when a node is just used for transactions and blocks.
  --no-rs is also accepted to disable the recursive DNS resolver (but keep the root server).

The default is false (meaning by default, hsd runs with the DNS servers).

[coveralls]

Pull Request Test Coverage Report for Build 680468365

• 24 of 26 (92.31%) changed or added relevant lines in 2 files are covered.
• 3 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+0.1%) to 59.708%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
lib/node/fullnode.js	12	13	92.31%
lib/node/spvnode.js	12	13	92.31%

Files with Coverage Reduction	New Missed Lines	%
lib/protocol/consensus.js	1	82.79%
lib/net/pool.js	2	30.27%

Totals
Change from base Build 676632891:	0.1%
Covered Lines:	19672
Relevant Lines:	30684

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💛 - Coveralls

[pinheadmz]

After discussion with @buffrr I think a better choice is keeping --no-dns but also adding --no-rs because in some configurations it may desirable to just run hsd as a root name server and use something externally (like BIND) for the recursive.

[nodech]

LGTM