Depending on when the context cancelation is picked up the handler may return and close the channel before `SubscribeJSON` returns, causing:
```
Feb 05 17:12:00 s3-us-node11 minio[3973657]: panic: send on closed channel
Feb 05 17:12:00 s3-us-node11 minio[3973657]: goroutine 378007076 [running]:
Feb 05 17:12:00 s3-us-node11 minio[3973657]: github.com/minio/minio/internal/pubsub.(*PubSub[...]).SubscribeJSON.func1()
Feb 05 17:12:00 s3-us-node11 minio[3973657]: github.com/minio/minio/internal/pubsub/pubsub.go:139 +0x12d
Feb 05 17:12:00 s3-us-node11 minio[3973657]: created by github.com/minio/minio/internal/pubsub.(*PubSub[...]).SubscribeJSON in goroutine 378010884
Feb 05 17:12:00 s3-us-node11 minio[3973657]: github.com/minio/minio/internal/pubsub/pubsub.go:124 +0x352
```
Wait explicitly for the goroutine to exit.
Bonus: Listen for doneCh when sending to not risk getting blocked there is channel isn't being emptied.
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