If the IMAP service happened to finish syncing and wanted to reset the
user event service at a time the latter was publishing an event a
deadlock would occur and the user would not receive any new messages.
This change puts the request to revert the event id in a separate
go-routine to avoid this situation from re-occurring. The operational
flow remains unchanged as the event service will only process this
request once the current set of events have been published.
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