Reduce slow DCB shutdown timeout to 2 seconds

This helps reduce the effect that a hanging connection has on the whole
system.

Making this configurable shouldn't be needed as long as the rate of
authentication failures doesn't exceed an acceptable level. In most
practical cases allowing two seconds for authentication to complete should
be enough.

server/core/routingworker.cc

@@ -165,9 +165,10 @@ void modules_thread_finish()
 
 bool can_close_dcb(DCB* dcb)
 {
+    const int SHOW_SHUTDOWN_TIMEOUT = 2;
     auto idle = MXS_CLOCK_TO_SEC(mxs_clock() - dcb->last_read);
 
-    return idle > 5                     // Timed out
+    return idle > SHOW_SHUTDOWN_TIMEOUT // Timed out
            || !dcb->func.can_close      // Not implemented
            || dcb->func.can_close(dcb); // Protocol says it's OK to close
 }