[avalanche] Make stale voting parameters configurable

Summary:
As we write more tests involving stale votes, functional testing times get too long.
This patch makes those voting parameters configurable and saves ~40 seconds when
running abc_p2p_avalanche_proof_voting on my machine.

This also gives fine tuned control to node operators, should it ever be necessary
to modify these parameters.

Test Plan:

ninja check check-functional

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: ecash1dollar, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D11548