MEDIUM: servers: Add a way to keep idle connections alive.

Add a new keyword for servers, "idle-timeout". If set, unused connections are
kept alive until the timeout happens, and will be picked for reuse if no
other connection is available.
diff --git a/include/types/connection.h b/include/types/connection.h
index cd9f1e7..745546c 100644
--- a/include/types/connection.h
+++ b/include/types/connection.h
@@ -444,7 +444,7 @@
 		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 */
-	struct timeval idle_tv;                 /* Time the connection was added to the idle list */
+	unsigned int idle_time;                 /* Time the connection was added to the idle list */
 };
 
 /* PROTO token registration */