jetstream: fix test_race failures

[AlexCuse]

We are seeing test_race failures intermittently with the jetstream package (more frequently in slower environments).

Example race detector output:

==================
WARNING: DATA RACE
Write at 0x00c0005805b0 by goroutine 316:
  runtime.closechan()
      /snap/go/current/src/runtime/chan.go:357 +0x0
  github.com/ThreeDotsLabs/watermill-nats/v2/pkg/jetstream.monitor()
      /home/alex/git/watermill-nats/pkg/jetstream/consumer.go:158 +0x104
  github.com/ThreeDotsLabs/watermill-nats/v2/pkg/jetstream.consume.func3()
      /home/alex/git/watermill-nats/pkg/jetstream/consumer.go:138 +0x6b

Previous read at 0x00c0005805b0 by goroutine 313:
  runtime.chansend()
      /snap/go/current/src/runtime/chan.go:160 +0x0
  github.com/ThreeDotsLabs/watermill-nats/v2/pkg/jetstream.(*Subscriber).handleMsg()
      /home/alex/git/watermill-nats/pkg/jetstream/message_handler.go:52 +0x595
  github.com/ThreeDotsLabs/watermill-nats/v2/pkg/jetstream.(*Subscriber).handleMsg-fm()
      <autogenerated>:1 +0x69
  github.com/ThreeDotsLabs/watermill-nats/v2/pkg/jetstream.consume.func1()
      /home/alex/git/watermill-nats/pkg/jetstream/consumer.go:132 +0x6c
  github.com/nats-io/nats.go/jetstream.(*pullConsumer).Consume.func1()
      /home/alex/go/pkg/mod/github.com/nats-io/nats.go@v1.31.0/jetstream/pull.go:228 +0x408
  github.com/nats-io/nats%2ego.(*Conn).waitForMsgs()
      /home/alex/go/pkg/mod/github.com/nats-io/nats.go@v1.31.0/nats.go:3045 +0x77e
  github.com/nats-io/nats%2ego.(*Conn).subscribeLocked.func1()
      /home/alex/go/pkg/mod/github.com/nats-io/nats.go@v1.31.0/nats.go:4256 +0x44

Goroutine 316 (running) created at:
  github.com/ThreeDotsLabs/watermill-nats/v2/pkg/jetstream.consume()
      /home/alex/git/watermill-nats/pkg/jetstream/consumer.go:138 +0x430
  github.com/ThreeDotsLabs/watermill-nats/v2/pkg/jetstream.(*Subscriber).Subscribe()
      /home/alex/git/watermill-nats/pkg/jetstream/subscriber.go:107 +0x3e4
  github.com/ThreeDotsLabs/watermill/pubsub/tests.TestContinueAfterErrors()
      /home/alex/go/pkg/mod/github.com/!three!dots!labs/watermill@v1.2.0/pubsub/tests/test_pubsub.go:764 +0x35b
  github.com/ThreeDotsLabs/watermill/pubsub/tests.TestPubSub.func1()
      /home/alex/go/pkg/mod/github.com/!three!dots!labs/watermill@v1.2.0/pubsub/tests/test_pubsub.go:76 +0x72
  github.com/ThreeDotsLabs/watermill/pubsub/tests.TestPubSub.runTest.func3.1()
      /home/alex/go/pkg/mod/github.com/!three!dots!labs/watermill@v1.2.0/pubsub/tests/test_pubsub.go:190 +0xe1
  testing.tRunner()
      /snap/go/current/src/testing/testing.go:1595 +0x238
  testing.(*T).Run.func1()
      /snap/go/current/src/testing/testing.go:1648 +0x44

Goroutine 313 (running) created at:
  github.com/nats-io/nats%2ego.(*Conn).subscribeLocked()
      /home/alex/go/pkg/mod/github.com/nats-io/nats.go@v1.31.0/nats.go:4256 +0x7cf
  github.com/nats-io/nats%2ego.(*Conn).subscribe()
      /home/alex/go/pkg/mod/github.com/nats-io/nats.go@v1.31.0/nats.go:4192 +0x137
  github.com/nats-io/nats%2ego.(*Conn).Subscribe()
      /home/alex/go/pkg/mod/github.com/nats-io/nats.go@v1.31.0/nats.go:4108 +0xace
  github.com/nats-io/nats.go/jetstream.(*pullConsumer).Consume()
      /home/alex/go/pkg/mod/github.com/nats-io/nats.go@v1.31.0/jetstream/pull.go:239 +0xa56
  github.com/ThreeDotsLabs/watermill-nats/v2/pkg/jetstream.consume()
      /home/alex/git/watermill-nats/pkg/jetstream/consumer.go:131 +0x1ac
  github.com/ThreeDotsLabs/watermill-nats/v2/pkg/jetstream.(*Subscriber).Subscribe()
      /home/alex/git/watermill-nats/pkg/jetstream/subscriber.go:107 +0x3e4
  github.com/ThreeDotsLabs/watermill/pubsub/tests.TestContinueAfterErrors()
      /home/alex/go/pkg/mod/github.com/!three!dots!labs/watermill@v1.2.0/pubsub/tests/test_pubsub.go:764 +0x35b
  github.com/ThreeDotsLabs/watermill/pubsub/tests.TestPubSub.func1()
      /home/alex/go/pkg/mod/github.com/!three!dots!labs/watermill@v1.2.0/pubsub/tests/test_pubsub.go:76 +0x72
  github.com/ThreeDotsLabs/watermill/pubsub/tests.TestPubSub.runTest.func3.1()
      /home/alex/go/pkg/mod/github.com/!three!dots!labs/watermill@v1.2.0/pubsub/tests/test_pubsub.go:190 +0xe1
  testing.tRunner()
      /snap/go/current/src/testing/testing.go:1595 +0x238
  testing.(*T).Run.func1()
      /snap/go/current/src/testing/testing.go:1648 +0x44

It seems like we are getting a callback delivered too late from the nats.go client, and in a way that existing synchronization code in the message handler doesn't catch. Test with updated nats.go client once a fix for nats-io/nats.go#1470 is available in case that helps.

[AlexCuse]

Note this fails almost every time in CI but not as often on my relatively powerful laptop. It is also rare that it fails on first run at all on the laptop, almost makes me wonder if repeated test runs are putting enough stress on the local broker / docker itself to create instability in the client. Hopefully we can find a reasonable synchronization solution between the two points but I thought the channel should be it :)