Serializability of Expected-Last-Subject-Sequence is not guaranteed in clustered stream

[bruth]

Defect

• Included nats-server -DV output
• Included a [Minimal, Complete, and Verifiable example] (https://stackoverflow.com/help/mcve)

Versions of nats-server and affected client libraries used:

Tested 2.9.15 - 2.9.20

OS/Container environment:

All

Steps or code to reproduce the issue:

Draft PR with a failing server test case.

Expected result:

Concurrent publishes having the same Nats-Expected-Last-Subject-Sequence should only result in one publish being accepted.

Actual result:

Multiple publishes may be accepted depending on timing.