fix(indexer): Fix concurrency issues in ReindexThread (#30083)

[spbolton]

Fixes concurrency issues and prevents race conditions that cause the indexer from hanging or shutting down immediately after startup.

This PR resolves #30083 (Startup timeouts due to indexer concurrency issues).

This PR fixes: #30083

[dcolina]

👍🏽

[wezell]

Added some comments - it is important to note that this is like touching a third rail.

I like a lot of these changes and where you are going with this Steve but my question to you would be what is the smallest PR that resolves the issue we are seeing now?

We will need to test a reindex in a cluster (3 node) with more content than the starter provides - something like 10000 pieces of content.

[wezell]

Important note - none of our ITs run against or expect a cluster to exist. This specifically needs to be tested by hand in a clustered configuration - 3 nodes - and test reindex starts, stops, switchovers, manual stops and manual switchovers.

[spbolton]

Modified this PR to a single change to hopefully fix the immediate issue and a follow up refactor work to fixup the general state management issues here #30083
#30110 as this will require more testing before it can be merged.

[spbolton]

@wezell Can we push in just this change, I think it will help on its own with the one issue here and we can spend some more time of general concurrency and state management issues in the new Issue I have created.

[spbolton]

Fixed sonarqube reported issue.
We were creating a "new Thread" that was not actually doing anything. We never started this thread object, it was working because thread itself extends Runnable so when passed to the Executor with submit it was just calling the run method of the "Runnable" we sent into the thread constructor. The thread name we set would not be used anywhere in the executor. We can just pass the original runnable getInstance().ReindexThreadRunnable directly into the submit method with the same effect. Creating the new Thread is both confusing and unecessary

[dotcms-sonarqube]

Quality Gate passed

Issues
0 New issues
1 Fixed issue
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube

[wezell]

Looks great.