linkcheck tests: use thread-safe collections.deque for client header recording

[jayaddison]

Feature or Bugfix

• Bugfix

Purpose

• When threaded test HTTP servers are in use, as in linkcheck builder: begin using session-based HTTP requests #11340, then client header related tests have been failing intermittently.
• This pull request attempts to address that by using a thread-safe collection that is shared between the unit test (object owner) and server handler (object recipient/producer thread).

Detail

• collections.deque is thread-safe and supports append and iteration, similar to the list object currently in-use.
• Tested as part of Testing: pr 11340 test flakiness jayaddison/sphinx#2.

Relates

• Resolves Flaky unit test: test_auth_header_uses_first_match #11348.

[jayaddison]

(please hold-off on merging until after the 6.2.0 release)

[jayaddison]

This does not seem like it'll provide a fix for #11348; see discussion in that issue for details.