During b9199e8b24
reivew it was suggested to use standard for loop
but when erasing element it causes increment on
invalid iterator, as role could have been erased
before.
This change brings back original code.
Fixes: https://github.com/scylladb/scylladb/issues/27422
Backport: no, offending commit not released yet
Closesscylladb/scylladb#27444
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.