rgw-multisite: why RGWRemoteDataLog::read_source_log_shards_next() return directly for mater zone?

[Xinying Song <songxinying.ftd@xxxxxxxxx>]

Hi cephers:
  I'm confusing about the output of `radosgw-admin sync status`
command. When running in the master zone, it only shows how many
shards are there that are late, without showing the timestamp of
oldest not applied log entry. Run this command on the slave zone will
show both num and timestamp information.
  The source code shows that is because
RGWRemoteDataLog::read_source_log_shards_next() function is doing
nothing and returns directly on master zone. What's the reason for
that?

Thanks!