Similar to what is done with blocks and txs, this remembers for each peer the time at which the last new (previously unknown) and valid proof is received.
This time is part of the peer stats and can be retrieved via the getpeerinfo RPC if avalanche is enabled.
The intent is to use that value for the peer eviction logic, but this is not part of this diff.

Depends on D9818.

Ref T1611.