Mirrors may remain out of sync for up to 15 minutes after upgrading to 6.5.0

Issue Summary

In Bitbucket 6.5.0 a new webhook was introduced: repo:mirror_synchronized. This webhook is triggered when a mirror has finished synchronizing with the upstream server. Mirrors subscribe to all repo:* events, but do not understand how to handle repo:mirror_synchronized and will therefore return a 400 Bad Request response to the upstream server when it tries to publish this event.
The upstream server has a circuit breaker for webhooks, which opens if a webhook fails too often. Since this circuit breaker opens for all repo:* events, even legitimate events, such as repo:refs_updated will not be published, and therefore the mirror will not receive any further events that would trigger it to synchronize with the upstream server.

Environment

Bitbucket Server 6.5.0

Steps to Reproduce

1. Upgrade upstream server to Bitbucket 6.5.0
2. Perform pushes to mirrored repositories frequently

Expected Results

• Mirror is updated within seconds after a push on the upstream server

Actual Results

• After a few pushes, the mirror stops synchronizing for up to 15 minutes, when a full sync is started.

The below exception is thrown in the atlassian-bitbucket.log file:

DEBUG c.a.b.i.m.m.r.w.DefaultRestWebHookEventConverter Unexpected server web hook key Optional[repo:mirror_synchronized] - dropping web hook payload

Workaround

Currently there is no known workaround for this behavior. A workaround will be added here when available