Add "perf" option to set starting diff based on algo-perf

[Spudz76]

• overrides default connection-port-based starting diff
• append "+perf" to login/user (fixed-diff)
• append "+perfauto" to login/user (auto-diff)
• append "+123456" for existing fixed-diff support
• anything else falls back to ignoring the "+" section (auto port-diff)

also if client does not send algo-perf in login packet, ignore/fallback to auto port-diff (for non-MO xmrig or meta-miner)

[MoneroOcean]

Looks OK. Will apply to jp.moneroocean.stream node first.