iurls OOBI resolution post-startup for witnesses to get past hanging OOBI resolution

[kentbull]

Feature request description/rationale

Until we get mailboxes removed from witnesses, and really as long as I have to support old style mailboxes, I need witness mailbox forwarding to be easier to test and use. It seems the easiest way to do this is to perform OOBI URL resolutions after the witness has started listening on its port, not before.

Currently in the witness startup process then any unreachable OOBI URL in the "iurls" property of the witness config causes witness startup to hang until it times out. Instead then the witness should start up, try the URL after startup, and report an error in the logs if the second attempt fails.

When trying to set up multiple witnesses at once that OOBI with each other on startup, via a Docker Compose file, then

• all of the witnesses hang because
• none of the other witnesses have started up
• all witnesses are hanging on their first OOBI resolution because no witness process is listening on it's configured port since it isn't fully started up yet.

The reason I need the witnesses OOBIing with each other is because witnesses must know where each other is at when processing /fwd messages between mailboxes for multisig events for group participants.

The solution to my problem seems to be to resolve the iurls after the witness has already started listening on its HTTP port because this would enable all OOBI resolutions between witnesses to complete.