In the following commits we want to move schema
commitlog replay earlier, but the current sstable
format should be selected before the replay.
The current sstable format is stored in system.scylla_local,
so we can't read it until system tables are loaded.
This problem is similar to the enabled_features.
To solve this we split sstables_format_selector in two
parts. The lower level part, sstables_format_selector,
knows only about database and system_keyspace. It
will be moved before system_keyspace initialization,
and the on_system_tables_loaded method will
be called on it when the system_keyspace has loaded its tables.
The higher level part, sstables_format_listener, is responsible
for subscribing to feature_services and gossipier and is started
later, at the same place as sstables_format_selector before this commit.
a0653590b5