[avalanche] Refactor the clearing of timed out requests in its own function

Summary: This makes the purpose of various pieces of code clearer. Because of the locks, it needs to scoped anyways.

Test Plan:

make check

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2473