MXS-1549: Prevent optimistic transaction with no slaves

If the router session does not have an already open slave connection,
optimistic transactions are not attempted.

server/modules/routing/readwritesplit/rwsplit_route_stmt.cc

@@ -151,13 +151,27 @@ void replace_binary_ps_id(GWBUF* buffer, uint32_t id)
 
 }
 
+bool RWSplitSession::have_connected_slaves() const
+{
+    for (const auto& b: m_backends)
+    {
+        if (b->is_slave() && b->in_use())
+        {
+            return true;
+        }
+    }
+
+    return false;
+}
+
 bool RWSplitSession::should_try_trx_on_slave(route_target_t route_target) const
 {
     return m_config.optimistic_trx &&        // Optimistic transactions are enabled
            !is_locked_to_master() &&         // Not locked to master
            !m_is_replay_active &&            // Not replaying a transaction
            m_otrx_state == OTRX_INACTIVE &&  // Not yet in optimistic mode
-           TARGET_IS_MASTER(route_target);  // The target type is master
+           TARGET_IS_MASTER(route_target) && // The target type is master
+           have_connected_slaves();          // At least one connected slave
 }
 
 bool RWSplitSession::track_optimistic_trx(GWBUF** buffer)

server/modules/routing/readwritesplit/rwsplitsession.hh

@@ -204,6 +204,9 @@ private:
 
     void handle_trx_replay();
 
+    // Do we have at least one open slave connection
+    bool have_connected_slaves() const;
+
     /**
      * Start the replaying of the latest transaction
      *