feat(channel): handle multiple subscribes partially in parallel

[keynslug]

Fixes EMQX-11483.

Release version: v5.5

Summary

If a SUBSCRIBE packet from the Client contains multiple subscriptions EMQX will now attempt to process them in parallel, at lest partially. More precisely: as before, Client will get a reply once the cluster (well, the set of core nodes) knows about new subscriptions, but the propagation of routing table updates per each new subscription will now happen concurrently if possible, i.e. when the batch_sync is enabled.

Currently includes #12237, will be rebased once it's merged.

PR Checklist

Please convert it to a draft if any of the following conditions are not met. Reviewers may skip over until all the items are checked:

• [x] Added tests for the changes
• [ ] ~~Added property-based tests for code which performs user input validation~~
• [ ] Changed lines covered in coverage report
• [ ] Change log has been added to changes/(ce|ee)/(feat|perf|fix|breaking)-<PR-id>.en.md files
• [x] For internal contributor: there is a jira ticket to track this change
• [ ] ~~Created PR to emqx-docs if documentation update is required, or link to a follow-up jira ticket~~
• [x] Schema changes are backward compatible