Whisper API fixed (#3687)

* whisper: wnode updated for tests with geth

* whisper: updated processing of incoming messages

* whisper: symmetric encryption updated

* whisper: filter id type changed to enhance security

* whisper: allow filter without topic for asymmetric encryption

* whisper: POW updated

* whisper: logging updated

* whisper: spellchecker update

* whisper: error handling changed

* whisper: JSON field names fixed