Sharded PubSub subscriptions not recovered after disconn… #2940

[ggivo]

…ection and re-connection.

Backport to 6.4.x branch

Make sure that:

• You have read the contribution guidelines.
• You have created a feature request first to discuss your contribution intent. Please reference the feature request ticket number in the pull request.
• You applied code formatting rules using the mvn formatter:format target. Don’t submit any formatting related changes.
• You submit test cases (unit or integration tests) that back your changes.