It's a simple helper used during boot-time that can enjoy query-processor from sharded<system_keyspace>
Closes#12587
* github.com:scylladb/scylladb:
system_keyspace: De-static system_keyspace::increment_and_get_generation
system_keyspace: Fix indentation after previous patch
system_keyspace: Coroutinize system_keyspace::increment_and_get_generation