MXS-852: Store the internal ID in the buffer

If the internal ID is stored in the buffer when it is moving inside the
readwritesplit router, the RWBackend can manage the execution of all
commands with a statement ID by replacing the stored ID with the correct
value.

server/core/session_command.cc

@@ -38,9 +38,12 @@ uint64_t SessionCommand::get_position() const
     return m_pos;
 }
 
-Buffer SessionCommand::copy_buffer() const
+GWBUF* SessionCommand::deep_copy_buffer()
 {
-    return m_buffer;
+    GWBUF* temp = m_buffer.release();
+    GWBUF* rval = gwbuf_deep_clone(temp);
+    m_buffer.reset(temp);
+    return rval;
 }
 
 SessionCommand::SessionCommand(GWBUF *buffer, uint64_t id):