Add mitigations for connection spam

[tehsausage]

• Adds HangupDelay config option (default 10 seconds) to automatically close uninitialized connections
• Adds QuietConnectionErrors config option (default off) to group together repeated connection failure log lines from the same IP
• Allows adjusting MaxConnections and PingRate via $rehash
• Fixes the MaxConnections check on client accept
• Removes the "too many connections to server" connection failure on init
• Closes uninitialized clients, starting from the oldest, to make room for new clients if possible