[Bug] Failed to create new subscription, got error "server error: ConsumerBusy: Exclusive consumer is already connected"

**Describe the bug**
Failed to create new subscription, got error "server error: ConsumerBusy: Exclusive consumer is already connected".

**To Reproduce**
Steps to reproduce the behavior:
- Create an exclusive topic;
- Create an subscription to this topic;
- Unsubscribe the topic after some time, but got error "connection closed";
- Then use new subscription to subscribe this topic, every try got "server error: ConsumerBusy: Exclusive consumer is already connected", Note: every new subscription differs from the first subscription;

**Expected behavior**
The new subscription should be created successfully.

**Screenshots**
If applicable, add screenshots to help explain your problem.

**Desktop (please complete the following information):**
- OS: [e.g. iOS]
- Version: 2.7.3
- Client Version: pulsar-client-go 0.6.1

**Additional context**
Add any other context about the problem here.

No repository file, test, or entry point is identified in the report. Start by reproducing the exclusive-topic subscription and unsubscribe sequence with pulsar-client-go 0.6.1, then trace the server-side consumer cleanup after the connection-closed error; done means a new subscription can connect successfully.

Written by the indexing model from the issue text.