Fix slave reconnection logic

Allowing calls to select_connect_backend_servers even when all slaves are
connected solves the debug assertion in select_connect_backend_servers
that happens when the execution of a queued query causes a new connection
to be created.

server/core/backend.cc

@@ -179,6 +179,7 @@ void Backend::set_state(backend_state state)
 
 bool Backend::connect(MXS_SESSION* session, SessionCommandList* sescmd)
 {
+    ss_dassert(!in_use());
     bool rval = false;
 
     if ((m_dcb = dcb_connect(m_backend->server, session, m_backend->server->protocol)))