Auth cache loading at startup is racing between
auth service and raft code and it doesn't support
concurrency causing it to crash.
We can't easily remove any of the places as during
raft recovery snapshot is not loaded and it relies
on loading cache via auth service. Therefore we add
semaphore.
Fixes https://github.com/scylladb/scylladb/issues/27540Closesscylladb/scylladb#27573
It combines data from all underlying auth tables.
Supports gentle full load and per role reloads.
Loading is done on shard 0 and then deep copies data
to all shards.