[avalanche] Bail when selecting a peer fails too many times

Summary:
This ensures we don't get stuck when there are no peers to select from, but entries remain in the slot list due to fragmentation.

Depends on D6809

Test Plan:

ninja all check-avalanche

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D6810