MXS-2025 RWBackends as a vector of unique_ptr:s

For lifetime management keep RWBackends in a vector of unique_ptrs.
RWSplitSession keeps the unique_ptrs very private, and provides a vector
of plain pointers for all other interfaces.

server/modules/routing/cat/cat.cc

@@ -43,7 +43,7 @@ CatSession* Cat::newSession(MXS_SESSION* pSession)
         }
     }
 
-    return connected ? new CatSession(pSession, this, backends) : NULL;
+    return connected ? new CatSession(pSession, this, std::move(backends)) : NULL;
 }
 
 void Cat::diagnostics(DCB* dcb)