[avalanche] Clear timed out proof poll requests

Summary:
As per title, demonstrated by reusing the existing unit test but with proofs.
Documentation for the explicit decorator declaration: https://www.boost.org/doc/libs/1_66_0/libs/test/doc/html/boost_test/tests_organization/decorators/explicit_decorator_declaration.html

Ref T1854.

Depends on D10271.

Test Plan:

ninja check-avalanche-processor_tests

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Maniphest Tasks: T1854

Differential Revision: https://reviews.bitcoinabc.org/D10277