db/view/view_building_worker: move discover_existing_staging_sstables() to the foreground
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 https://github.com/scylladb/scylladb/issues/26403).
Fixes https://github.com/scylladb/scylladb/issues/26417
The patch should be backported to 2025.4
Closesscylladb/scylladb#26446
* github.com:scylladb/scylladb:
db/view/view_building_worker: move discover_existing_staging_sstables() to the foreground
db/view/view_building_worker: futurize and rename `start_background_fibers()`
0b800aab17