Fixed deadlock in DLQ ack processing

[merlimat]

Motivation

When acking the original message, after sending to the DLQ, there is a potential deadlock if the producer is sharing the same connection as the consumer. The problem is we're trying to write on the connection event channel from the same go-routine that's handling the connection.

1 @ 0x4038f00 0x400774d 0x4007515 0x4606e8d 0x4613487 0x4603456 0x461cf4c 0x4619ab8 0x45f8bde 0x45f805e 0x45f737d 0x45fed61 0x4068bb1
#	0x4606e8c	github.com/apache/pulsar-client-go/pulsar.(*partitionConsumer).AckID+0x25c			/Users/mmerli/go/src/github.com/apache/pulsar-client-go/pulsar/consumer_partition.go:279
#	0x4613486	github.com/apache/pulsar-client-go/pulsar.trackingMessageID.Ack+0x106				/Users/mmerli/go/src/github.com/apache/pulsar-client-go/pulsar/impl_message.go:55
#	0x4603455	github.com/apache/pulsar-client-go/pulsar.(*consumer).AckID+0x165				/Users/mmerli/go/src/github.com/apache/pulsar-client-go/pulsar/consumer_impl.go:371
#	0x461cf4b	github.com/apache/pulsar-client-go/pulsar.(*dlqRouter).run.func1+0x19b				/Users/mmerli/go/src/github.com/apache/pulsar-client-go/pulsar/dlq_router.go:104
#	0x4619ab7	github.com/apache/pulsar-client-go/pulsar.(*partitionProducer).ReceivedSendReceipt+0x3a7	/Users/mmerli/go/src/github.com/apache/pulsar-client-go/pulsar/producer_partition.go:510
#	0x45f8bdd	github.com/apache/pulsar-client-go/pulsar/internal.(*connection).handleSendReceipt+0xed		/Users/mmerli/go/src/github.com/apache/pulsar-client-go/pulsar/internal/connection.go:599
#	0x45f805d	github.com/apache/pulsar-client-go/pulsar/internal.(*connection).internalReceivedCommand+0x1cd	/Users/mmerli/go/src/github.com/apache/pulsar-client-go/pulsar/internal/connection.go:518
#	0x45f737c	github.com/apache/pulsar-client-go/pulsar/internal.(*connection).run+0x33c			/Users/mmerli/go/src/github.com/apache/pulsar-client-go/pulsar/internal/connection.go:378
#	0x45fed60	github.com/apache/pulsar-client-go/pulsar/internal.(*connection).start.func1+0x70		/Users/mmerli/go/src/github.com/apache/pulsar-client-go/pulsar/internal/connection.go:231