don't send a version negotiation packet in response to a version negotiation packet

[marten-seemann]

Annoyingly, this is the bug in the QUIC spec I had discovered for the Tokyo interim meeting back in Jan 2017.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@5d2b87e). Click here to learn what that means.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##             master    #2818   +/-   ##
=========================================
  Coverage          ?   85.94%           
=========================================
  Files             ?      133           
  Lines             ?    12107           
  Branches          ?        0           
=========================================
  Hits              ?    10405           
  Misses            ?     1367           
  Partials          ?      335           

Impacted Files	Coverage Δ
server.go	81.45% <100.00%> (ø)

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