Ensure a given UAID is still valid when trying to register an endpoint

[data-sync-user]

There are a few reports of users registering an endpoint which is immediately invalid see bugzilla. It may be that the server is maintaining a connection it should not be.

┆Issue is synchronized with this Jira Task

[data-sync-user]

➤ Philip Jenvey commented:

I’ll reiterate my older comment ( #713 (comment) ) here: this is lower priority, as this looks more likely like a client bug than server – client behavior should be ruled out before any server changes.