Investigate issue with duplicate cruise start mails

[AudunVN]

I suspect this might be a cruise that was submitted and subsequently edited before the recent patches to the server to fix this issue.

[AudunVN]

Issue is related to database blocking during email send, which results in an error and the email send job retrying multiple times before it's able set the is_sent field to true.

[AudunVN]

Should be resolved by latest commits (new DB structure, less expensive email send checks, scheduling changes). Will likely be deployed to production this weekend.

[AudunVN]

Still happening.