ISSUE-7455: Key_Shared consumers with different subscriptions get messages out-of-order in 2.6.0.

[sijie]

Original Issue: apache#7455

---

Describe the bug
Key_Shared consumers with different subscriptions get messages out-of-order in 2.6.0.
As far as I checked, it doesn't happen with Consistent Hashing approach.

To Reproduce
Steps to reproduce the behavior:
https://github.com/feeblefakie/misc/blob/master/pulsar/HOW-TO-REPRODUCE.md

Expected behavior
Key_Shared consumers with different subscriptions should get messages in a consistent order to meet the ordering guarantee of Key_Shared.

Desktop (please complete the following information):

• MacOSX (it could happen in a single node testing)

Additional context

I reported the bug in 2.5.0 and it seems fixed by the following PRs, but it is still happening.
http://mail-archives.apache.org/mod_mbox/pulsar-users/202003.mbox/%3CCAPDOW74LN3WtdhpG_cgCCSg9MuMmkNV6giCGD5p%3DW1wWji0W7w%40mail.gmail.com%3E

apache#6554
apache#6977
apache#7106