When heartbeat failed, we should clear the connections cached
in client pool, or we will get broken connections from the pool.
Since we don't have the REOPEN logic(which may cause ugly code style),
a broken connection may cause a rpc blocked and failed.
So clear them all and recreate them when needed is a simple way to
resolve this problem.
We only clear connections in backend and broker pool.
No need to clear heartbeat pool because heartbeat is very frequent,
such the connections can be invalid automatically.
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