whisper: message format refactoring (#14335)

* whisper: salt removed from AES encryption
* whisper: padding format updated
* whisper: padding test added
* whisper: padding refactored, tests fixed
* whisper: padding test updated
* whisper: wnode bugfix
* whisper: send/receive protocol updated
* whisper: minor update
* whisper: bugfix in test
* whisper: updated parameter names and comments
* whisper: functions renamed
* whisper: minor refactoring