MEDIUM: servers: Add a command to limit the number of idling connections.

Add a new command, "pool-max-conn" that sets the maximum number of connections
waiting in the orphan idling connections list (as activated with idle-timeout).
Using "-1" means unlimited. Using pools is now dependant on this.
diff --git a/include/types/connection.h b/include/types/connection.h
index bcc2965..eff1001 100644
--- a/include/types/connection.h
+++ b/include/types/connection.h
@@ -446,7 +446,7 @@
 		struct sockaddr_storage from;	/* client address, or address to spoof when connecting to the server */
 		struct sockaddr_storage to;	/* address reached by the client, or address to connect to */
 	} addr; /* addresses of the remote side, client for producer and server for consumer */
-	unsigned int idle_time;                 /* Time the connection was added to the idle list */
+	unsigned int idle_time;                 /* Time the connection was added to the idle list, or 0 if not in the idle list */
 };
 
 /* PROTO token registration */