#14683 Serve liveness requests on the gRPC server thread

Requests are queued by the completion queue thread and processed from the main
thread. A long running request, or an open progress dialog in the GUI, therefore
left every request unanswered for as long as that operation lasted, including
the liveness checks a client uses to decide whether ResInsight is still there.

Let a callback declare that it touches no application state, and serve those
directly on the completion queue thread instead of queueing them. GetVersion is
the only such callback, since it reads compile-time constants only. Everything
that touches the project or the data model is still processed on the main
thread, so no request can observe or modify the data model while another one is
running.

The allocated callback list is now used from both threads and is guarded by a
mutex. Shutdown takes a lock before flagging that it is in progress, so that it
waits for any dispatch already running on the completion queue thread and no new
request handler can be registered on a queue that is about to be shut down.

During a blocking call of 3.25 s, 65 pings were answered with a maximum latency
of 6.2 ms. Before this change none of them were answered until the call
completed.

GrpcInterface/RiaGrpcCallbacks.inl

@@ -93,11 +93,13 @@ ReplyT& RiaGrpcServiceCallback<ServiceT, RequestT, ReplyT>::reply()
 template <typename ServiceT, typename RequestT, typename ReplyT>
 RiaGrpcUnaryCallback<ServiceT, RequestT, ReplyT>::RiaGrpcUnaryCallback( ServiceT*      service,
                                                                         MethodImplT    methodImpl,
-                                                                        MethodRequestT methodRequest )
+                                                                        MethodRequestT methodRequest,
+                                                                        bool           runsOnServerThread )
     : RiaGrpcServiceCallback<ServiceT, RequestT, ReplyT>( service )
     , m_responder( &m_context )
     , m_methodImpl( methodImpl )
     , m_methodRequest( methodRequest )
+    , m_runsOnServerThread( runsOnServerThread )
 {
 }
 
@@ -107,7 +109,10 @@ RiaGrpcUnaryCallback<ServiceT, RequestT, ReplyT>::RiaGrpcUnaryCallback( ServiceT
 template <typename ServiceT, typename RequestT, typename ReplyT>
 RiaGrpcCallbackInterface* RiaGrpcUnaryCallback<ServiceT, RequestT, ReplyT>::createNewFromThis() const
 {
-    return new RiaGrpcUnaryCallback<ServiceT, RequestT, ReplyT>( this->m_service, this->m_methodImpl, this->m_methodRequest );
+    return new RiaGrpcUnaryCallback<ServiceT, RequestT, ReplyT>( this->m_service,
+                                                                 this->m_methodImpl,
+                                                                 this->m_methodRequest,
+                                                                 m_runsOnServerThread );
 }
 
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