This patch moves `discover_existing_staging_sstables()` to be executed
from main level, instead of running it on the background fiber.
This method need to be run only once during the startup to collect
existing staging sstables, so there is no need to do it in the
background. This change will increase debugability of any further issues
related to it (like scylladb/scylladb#26403).
Fixesscylladb/scylladb#26417
84e4e34d81