Multiple concurrent gateway tunnel close requests only closes the first tunnel #2424

Describe the bug
When multiple tunnels are closed at the same time the edge gateway only seems to process the first request; the backend then assumes all the tunnels have been closed which is incorrect so a tunnel is left open indefinitely,

To Reproduce
Steps to reproduce the behavior:

1. Setup manager instance with tunnelling support
2. Configure edge gateway with SSH key to enable tunnelling
3. open multiple tunnels on the gateway
4. Close them all at the same time (might require some UI hacking to close all very quickly)
5. Look at SISH container logs and only the first tunnel will be closed

Expected behavior
All tunnels should be closed, if a tunnel doesn't actually close then it should still show in the central manager tunnel list.