Close client DCB with a hangup in the backend protocol

Directly closing the client DCB in the backend protocol modules is not
correct anymore as the state of the session doesn't change when the client
DCB is closed. By propagating the shutdown of the session with a fake
hangup to the client DCB, the closing of the DCB is done only once.

Added debug assertions that make sure all DCBs are closed only
once. Removed redundant code in the backend protocol error handling code.

server/core/backend.cc

@@ -43,6 +43,8 @@ Backend::~Backend()
 
 void Backend::close(close_type type)
 {
+    ss_dassert(m_dcb->n_close == 0);
+
     if (!m_closed)
     {
         m_closed = true;