[draft] Fix setting cursor position on NonDurable subcription

[michalcukierman]

PR is a draft and contains only failing test.

Motivation

After re-creating NonDurable subscription, the cursor was moved back by 1 position.
This should only happen when there is no startMessageId provided (see tests in the original PR).
Broker does not store batch index information, therefore it has to be provided while subscription initialization.

Before this fix, the backlog is incorrect, and the batch message later ignored by the client.

Modifications

Added startMessageId != null check, to ensure that the we only set relative position in a batch, when we have this information.

Verifying this change

• Make sure that the change passes the CI checks.

This change is already covered by existing tests, such as (please describe tests).

• Broker tests where covering the original case. I've added one more test to cover this case.

Does this pull request potentially affect one of the following parts:

If the box was checked, please highlight the changes

• Dependencies (add or upgrade a dependency)
• The public API
• The schema
• The default values of configurations
• The threading model
• The binary protocol
• The REST endpoints
• The admin CLI options
• The metrics
• Anything that affects deployment

Documentation

• doc
• doc-required
• doc-not-needed
• doc-complete

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[michalcukierman]

Closing as the code was already copied in another PR.