json_db: exempt keystore from StoredDict conversion

The keystore logic would need to be significantly changed to nicely
interoperate with StoredDict/json_db logic.
(just see KeyStore.__init__() and KeyStore.dump())
For now we exempt the keystore from the recursive StoredDict conversion, as
it is a smaller change that is also easier to review for correctness.

fixes #6066
fixes #6401

also reverts 2d3c2eeea9 (which was an even hackier workaround for #6066)

electrum/keystore.py

@@ -894,7 +894,6 @@ def hardware_keystore(d) -> Hardware_KeyStore:
 
 def load_keystore(db: 'WalletDB', name: str) -> KeyStore:
     d = db.get(name, {})
-    d = dict(d)  # convert to dict from StoredDict (see #6066)
     t = d.get('type')
     if not t:
         raise WalletFileException(