[avalanche] Correctly flag the inflight compact proofs request

Summary: The periodic requests are not flag properly as inflight and subsequent responses are so ignored.

Test Plan:

./test/functional/test_runner.py abc_p2p_compactproofs

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D11692