system_keyspace: mark scylla_local table as always-sync commitlog

It is infrequently updated (typically once at start) but stores
critical state for this instance survival (Raft Group 0 id, Raft
server id, sstables format), so always write it to commit log
in sync mode.

db/system_keyspace.cc

@@ -649,6 +649,8 @@ schema_ptr system_keyspace::large_cells() {
         "Scylla specific information about the local node"
        );
        builder.set_gc_grace_seconds(0);
+       // Raft Group id and server id updates must be sync
+       builder.set_wait_for_sync_to_commitlog(true);
        builder.with_version(generate_schema_version(builder.uuid()));
        return builder.build(schema_builder::compact_storage::no);
     }();

db/system_keyspace.hh

@@ -204,7 +204,7 @@ public:
         static schema_ptr batchlog();
     };
 
-    static constexpr const char* extra_durable_tables[] = { PAXOS };
+    static constexpr const char* extra_durable_tables[] = { PAXOS, SCYLLA_LOCAL };
 
     static bool is_extra_durable(const sstring& name);