Add host_tx_ready enhancements (#2930)

What I did
Add host_tx_ready enhancements to sonic-swss branch according to HLD

Why I did it
Module and Asic configurations must be synchronized in order to not configure an non-existing module.

orchagent/main.cpp

@@ -505,6 +505,10 @@ int main(int argc, char **argv)
     attr.value.ptr = (void *)on_switch_shutdown_request;
     attrs.push_back(attr);
 
+    attr.id = SAI_SWITCH_ATTR_PORT_HOST_TX_READY_NOTIFY;
+    attr.value.ptr = (void *)on_port_host_tx_ready;
+    attrs.push_back(attr);
+
     if (gMySwitchType != "fabric" && gMacAddress)
     {
         attr.id = SAI_SWITCH_ATTR_SRC_MAC_ADDRESS;

orchagent/notifications.cpp

@@ -47,3 +47,9 @@ void on_switch_shutdown_request(sai_object_id_t switch_id)
 
     quick_exit(EXIT_FAILURE);
 }
+
+void on_port_host_tx_ready(sai_object_id_t switch_id, sai_object_id_t port_id, sai_port_host_tx_ready_status_t m_portHostTxReadyStatus)
+{
+    // don't use this event handler, because it runs by libsairedis in a separate thread
+    // which causes concurrency access to the DB
+}

orchagent/notifications.h

@@ -11,3 +11,4 @@ void on_bfd_session_state_change(uint32_t count, sai_bfd_session_state_notificat
 // The function prototype information can be found here:
 //      https://github.com/sonic-net/sonic-sairedis/blob/master/meta/NotificationSwitchShutdownRequest.cpp#L49
 void on_switch_shutdown_request(sai_object_id_t switch_id);
+void on_port_host_tx_ready(sai_object_id_t switch_id, sai_object_id_t port_id, sai_port_host_tx_ready_status_t m_portHostTxReadyStatus);