UI update can be triggered twice

[RobinTail]

the UI update can be triggered twice (by action and by polling).
It needs a lock.

2023-08-16 23:05:21,643 - octoprint.plugins.octorelay - INFO - Received the API command update with parameters: {'pin': 'r3', 'command': 'update'}
2023-08-16 23:05:21,643 - octoprint.plugins.octorelay - DEBUG - Requested to switch the relay r3
2023-08-16 23:05:21,759 - octoprint.plugins.octorelay - DEBUG - Toggling the relay r3 on pin 18
2023-08-16 23:05:21,760 - octoprint.plugins.octorelay - DEBUG - Updating the UI
2023-08-16 23:05:21,799 - octoprint.plugins.octorelay - DEBUG - relay: r3 has changed its pin state
2023-08-16 23:05:21,799 - octoprint.plugins.octorelay - DEBUG - Updating the UI

improved logging helped to reveal it.

Originally posted by @RobinTail in #183 (comment)