Move large query processing inside RWBackend

The knowledge of which function to call can be internal to RWBackend. This
make the use of the class easier as one can simply write to the backend.

server/modules/protocol/MySQL/rwbackend.cc

@@ -75,6 +75,15 @@ uint32_t RWBackend::get_ps_handle(uint32_t id) const
 
 bool RWBackend::write(GWBUF* buffer, response_type type)
 {
+    uint32_t len = mxs_mysql_get_packet_len(buffer);
+    bool was_large_query = m_large_query;
+    m_large_query = len == MYSQL_PACKET_LENGTH_MAX;
+
+    if (was_large_query)
+    {
+        return mxs::Backend::write(buffer, Backend::NO_RESPONSE);
+    }
+
     if (type == mxs::Backend::EXPECT_RESPONSE)
     {
         /** The server will reply to this command */