fix(ecpool,bridge): treat {error, ecpool_empty} as a retriable error
Example from flaky CI test:
```
/emqx/apps/emqx_resource/src/emqx_resource_buffer_worker.erl:716 2023-06-01T19:30:37.119862+00:00 [buffer_worker_flush_ack] #{batch_or_query => [{query,undefined,{send_message,#{payload => <<"-576460752303420927">>,timestamp => 1668602148000,topic => <<"t_write_failure">>}},false,-576460614954536809}],queue_count => 0,result => {async_return,{error,{unrecoverable_error,ecpool_empty}}},'~meta' => #{gl => <0.3883.5>,node => 'test@127.0.0.1',pid => <0.4724.5>}}.
```
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