Race condition and inconsistencies in ConfigDBConnector_Native::db_connect

[huanlev]

We observe the following issues in ConfigDBConnector_Native::db_connect():

1. client.get(INIT_INDICATOR) is called before psubscribe(). Hence, it is possible for a client getting a stale INIT_INDICATOR value and miss the keyspace notification on subsequent update. Suggest to psubscribe() before reading the INIT_INDICATOR instead.
2. initialized->empty() is used to check for the blocking/returning condition which is inconsistent with the check in docker_image_ctl.j2 in sonic-buildimage. Would this *initialized != "0" check be more accurate?

If these changes are reasonable, I can create a PR for them.