Currently when we set a single value we need
to call broadcast_to_all_shards to let observers on all
shards get notified of the new value.
However, the latter broadcasts all value to all shards
so it's terribly inefficient.
Instead, add async set_value_on_all_shards functions
to broadcast a value to all shards.
Use those in system_keyspace for db_config_table virtual table
and in task_manager_test to update the task_manager ttl.
Refs #7316
Signed-off-by: Benny Halevy <bhalevy@scylladb.com>
1a183047c0