[WIP] Move dns_instrument from onCompleted to onHeadersReceived

[englehardt]

Fixes #1020 WIP. I haven't tested this much.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 45.74%. Comparing base (7b8cf99) to head (8a855a4).
⚠️ Report is 35 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1021   +/-   ##
=======================================
  Coverage   45.74%   45.74%           
=======================================
  Files          35       35           
  Lines        3478     3478           
=======================================
  Hits         1591     1591           
  Misses       1887     1887           

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[vringar]

Good Catch! Does this also resolve the issue where DNS requests for domains that don't resolved aren't captured?

[englehardt]

I'm not sure. I'll need to write a test page for both of these conditions.