[WIP] Logging, auditing, warnings, and PingContext failure handling for switch DB

[ninabauer]

• adds log output when switching db name
• adds dbname in audit data
• after OverrideDBName, try a DB.PingContext, if it fails, fall back to its last value.
• when current db name is different with env DB_NAME, returns a warning saying current db is not the default one in the response json

[BumbleFeng]

Is there any particular reason to set Warnings as an array？ It is a bit weird to have Error as string but Warning as Array.

[BumbleFeng]

Could you add dbName and defaultDBName info to the warn msg?