ISSUE-281: producer.Send() blocks for too long time

[sijie]

Original Issue: apache#281

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goroutine 102748631 [semacquire, 11167 minutes]:
sync.runtime_Semacquire(0xc026c4b9d8)
        /usr/local/go/src/runtime/sema.go:56 +0x39
sync.(*WaitGroup).Wait(0xc026c4b9d0)
        /usr/local/go/src/sync/waitgroup.go:130 +0x65
github.com/apache/pulsar-client-go/pulsar.(*partitionProducer).Send(0xc005e5dd60, 0xe61760, 0xc00fc75080, 0xc0266fbc20, 0xbfaf54b1cfe715d9, 0x556c4fd0d57d, 0x1497240, 0xe61760)
        /Users/faker/go/pkg/mod/github.com/apache/[email protected]/pulsar/producer_partition.go:376 +0x171
github.com/apache/pulsar-client-go/pulsar.(*producer).Send(0xc0062611a0, 0xe61760, 0xc00fc75080, 0xc0266fbc20, 0xc00fc75080, 0xc01dc51d40, 0xc008babe10, 0x4055a5)

By visiting /debug/pprof/goroutine?debug=2 to get the running status of goutoutines, I found that some producer.Send() has been blocked and cannot be released.

Is there a way to solve it?