[avalanche] Add stake contenders to the cache every time a proof is registered

Summary:
This is a more robust way of adding contenders to the cache since proofs can be registered in many ways. Immature proofs delay their registration until their utxos mature. Persisted proofs are loaded and registered immediately on restart. Proofs can also be added manually via RPC. Adding them as contenders during registration captures all of these (and any future) cases.

The only downside with this approach is a proof not coming from the network may not be marked as remote until peers see it too. Since contenders cannot be promoted unless they are also remote, we have to ensure that proofs arriving ex-network get re-added to the cache in case they were skipped over for promotion.

Test Plan:

for I in {0..20} ; do ./test/functional/test_runner.py abc_p2p_avalanche_contender_voting || break ; done

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D17867