[avalanche] refactor getInvsForNextPoll to limit the scope of cs_main

Summary: It's a good idea to limit the scope over which you hold cs_main.

Test Plan:

make check

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2472