ISSUE-644: panic: send on closed channel

[sijie]

Original Issue: apache#644

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System configuration

Pulsar version: 2.7.2
client-go :master

2288469 Oct 20 22:07:46 VM-193-17-centos gateway: panic: send on closed channel
2288470 Oct 20 22:07:46 VM-193-17-centos gateway: goroutine 736382836 [running]:
2288471 Oct 20 22:07:46 VM-193-17-centos gateway: github.com/apache/pulsar-client-go/pulsar/internal.(*connection).SendRequest(0xc01a12c160, 0        x3, 0xc019a42a80, 0xc01f2f9b80)
2288472 Oct 20 22:07:46 VM-193-17-centos gateway: github.com/apache/pulsar-client-go@v0.6.0/pulsar/internal/connection.go:578 +0x1e7
2288473 Oct 20 22:07:46 VM-193-17-centos gateway: github.com/apache/pulsar-client-go/pulsar/internal.(*rpcClient).Request(0xc01470cea0, 0x0, 0        x0, 0x0, 0x0, {0x1314f40, 0xc00bb34300})
2288474 Oct 20 22:07:46 VM-193-17-centos gateway: github.com/apache/pulsar-client-go@v0.6.0/pulsar/internal/rpc_client.go:127 +0x152
2288475 Oct 20 22:07:46 VM-193-17-centos gateway: github.com/apache/pulsar-client-go/pulsar.(*partitionConsumer).grabConn(0xc01c544340)
2288476 Oct 20 22:07:46 VM-193-17-centos gateway: github.com/apache/pulsar-client-go@v0.6.0/pulsar/consumer_partition.go:975 +0xd28
2288477 Oct 20 22:07:46 VM-193-17-centos gateway: github.com/apache/pulsar-client-go/pulsar.newPartitionConsumer({0x1331320, 0xc01586a1a0}, 0x        c01470cde0, 0xc018520e00, 0xc01470cfc0, 0xc019996e60, 0xc00b020900)
2288478 Oct 20 22:07:46 VM-193-17-centos gateway: github.com/apache/pulsar-client-go@v0.6.0/pulsar/consumer_partition.go:184 +0x6a6
2288479 Oct 20 22:07:46 VM-193-17-centos gateway: github.com/apache/pulsar-client-go/pulsar.(*consumer).internalTopicSubscribeToPartitions.fun        c1(0x0, {0xc019bce4d0, 0x65})
2288480 Oct 20 22:07:46 VM-193-17-centos gateway: github.com/apache/pulsar-client-go@v0.6.0/pulsar/consumer_impl.go:341 +0x2d6
2288481 Oct 20 22:07:46 VM-193-17-centos gateway: created by github.com/apache/pulsar-client-go/pulsar.(*consumer).internalTopicSubscribeToPar        titions
2288482 Oct 20 22:07:46 VM-193-17-centos gateway: github.com/apache/pulsar-client-go@v0.6.0/pulsar/consumer_impl.go:313 +0x3d6