dns/server: return KEY rr

[tynes]

This PR returns a KEY resource record containing the SIG(0) key. This is useful for integrating with systems that expect to be able to validate SIG(0) and automatically look for a KEY rr. This is an alternative to #299 and is meant to make applications like https://github.com/james-stevens/handshake-bridge more simple to put together.

[coveralls]

Pull Request Test Coverage Report for Build 203068515

• 4 of 4 (100.0%) changed or added relevant lines in 1 file are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.005%) to 58.98%

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

Totals
Change from base Build 202553015:	0.005%
Covered Lines:	19289
Relevant Lines:	30410

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💛 - Coveralls