fix: introduce a new config key to store an atsign's blocklist

[srieteja]

Closes #1570 and #1204

- What I did

• introduce a new config key to store an at-sign's blocklist
• allow backward compatibility to ensure data stored under the old key is smoothly migrated to the new key
• ensure the new key is not synced to clients
• delete the old key after data migration, which is actually the cause of the issue linked above
• remove an unnecessary try-catch block in ConfigVerbHandler
• Replace assert statements in AtConfig with IllegalArgumentExceptions
• Group all the tests in at_persistence_secondary/at_config_test. Also added code to close the hive in the tearDown.

- How I did it

• New key that stores an at-sign's blocklist is: 'private:blocklist@atsign'.
• This key name is more descriptive of its function and ensures that this key is not synced to the client(Keys that start with 'private:' are not synced).
• When the new key is not found in the keystore, an attempt to fetch data with the old key is made. And then the existing data is stored with the new config-key while the old one is deleted.

- How to verify it

• added unit test to objectively verify backward compatibility
• existing unit and functional tests passing should indicate the proper working of this feature

- Description for the changelog

fix: introduce a new config key to store an at-sign's blocklist