[avalanche] Limit the number of inflight proof poll requests

Summary:
Apply the limit of 10 inflight proof poll request (the same limit that applies to blocks). The count increment is skipped when the forPoll flag is set (used in tests).

Ret T1854.

Depends on D10277.

Test Plan:

ninja check-avalanche-processor_tests

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D10278