The items are re-indexing on each cron run

I created a patch which fixes this issue.

I created a mechanism to check if any strategy was added/removed, and only then the index is flagged for re-indexing.

Also changed the hook_cron to hook_rebuild. When a new strategy is created and added to the service, a cache rebuild is required. So it's enough to run the Sync service only then, not on cron.

Accidentally I left some debugging in the first patch. Here is a new one