[redis] Design limitation of Configdbconnector could lead to a blackout when using listen

[vivekrnv]

Description

Exact issue seen in the hostcfgd is documented here #8619. (Hostcfgd using configdbconnector is the reason here)

To mitigate this either config_db.listen() can be modified to read the existing data and treat them like an incoming message.

or the usage of ConfigDBConnector connector can be replaced by SubscriberStateTable which already follows this design

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[dgsudharsan]

@qiluo-msft @renukamanavalan FYI

[dgsudharsan]

@zhangyanzhao This is a documentation of existing limitation we found with Configdbconnector as requested by @qiluo-msft . Community must plan to move from using configdbconnector to SubscriberStateTable on existing Daemons.

[vivekrnv]

Fixed by sonic-net/sonic-swss-common#587