Fix critical bugs and security vulnerabilities

- Fix race condition in client cleanup by serializing operations
- Add proper nil checks in SendMessage for server/config
- Add semaphore to limit concurrent health check goroutines
- Reduce buffer size to RFC-compliant 512 bytes (was 4096)
- Add comprehensive input validation (length, null bytes, UTF-8)
- Improve SSL error handling with graceful degradation
- Replace unsafe conn.Close() with proper cleanup() calls
- Prevent goroutine leaks and memory exhaustion attacks
- Enhanced logging and error recovery throughout

These fixes address the freezing issues and improve overall
server stability, security, and RFC compliance.

commands.go

@@ -1096,11 +1096,8 @@ func (c *Client) handleQuit(parts []string) {
 		}
 	}
 
-	// Remove client from server
-	c.server.RemoveClient(c)
-
-	// Close the connection
-	c.conn.Close()
+	// Use proper cleanup instead of direct connection close
+	c.cleanup()
 }
 
 // handleMode handles MODE command
@@ -2080,8 +2077,8 @@ func (c *Client) handleKill(parts []string) {
 		}
 	}
 
-	// Disconnect the target
-	target.conn.Close()
+	// Disconnect the target properly
+	target.cleanup()
 }
 
 // handleOper handles OPER command