[avalanche] Fix a lock order inversion

Summary: Introduced in D17832, we are calling addToReconcile() which in turns calls IsWorthPolling() with cs_main hold, which is not possible.

Test Plan: Run the TSAN build.

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D17971