Support multiple proxies in one NaiveProxy process to save memory usage.

Examples:

• "listen": ["http://:8080", "http://:8081"]: Listens on port 8080 and 8081; both route to origin directly.
• "listen": ["http://:8080", "http://:8081"], "proxy": "https://a.com": Listens on port 8080 and 8081; both route to the proxy server a.com.
• "listen": ["http://:8080", "http://:8081"], "proxy": ["https://a.com", "https://b.com"]: Listens on port 8080 and 8081. Port 8080 routes to a.com. Port 8081 routes to b.com.

Note that --proxy=https://a.com,https://b.com is a proxy chain, not two proxies.