BUG/MEDIUM: email-alert: don't set server check status from a email-alert task

BUG/MEDIUM: email-alert: don't set server check status from a email-alert task

This avoids possible 100% cpu usage deadlock on a EMAIL_ALERTS_LOCK and
avoids sending lots of emails when 'option log-health-checks' is used.
It is avoided to change the server state and possibly queue a new email
while processing the email alert by setting check->status to
HCHK_STATUS_UNKNOWN which will exit the set_server_check_status(..) early.

This needs to be backported to 1.8.