* vmalert: fix sending alert messages
1. fix `endsAt` field in messages that send to alertmanager, previously rule with small interval could never be triggered;
2. fix behavior of `-rule.resendDelay`, before it could prevent sending firing message when rule state is volatile.
Signed-off-by: hagen1778 <roman@victoriametrics.com>
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Signed-off-by: hagen1778 <roman@victoriametrics.com>
Co-authored-by: hagen1778 <roman@victoriametrics.com>
(cherry picked from commit d7224b2d1c)
Signed-off-by: hagen1778 <roman@victoriametrics.com>