Separate WalletDB from storage upgrades.

Make sure that WalletDB.data is always a StoredDict.
Perform db upgrades in a separate class, since they
operate on a dict object.

electrum/daemon.py

@@ -47,7 +47,7 @@ from .util import log_exceptions, ignore_exceptions, randrange, OldTaskGroup
 from .util import EventListener, event_listener
 from .wallet import Wallet, Abstract_Wallet
 from .storage import WalletStorage
-from .wallet_db import WalletDB
+from .wallet_db import WalletDB, WalletRequiresSplit, WalletRequiresUpgrade
 from .commands import known_commands, Commands
 from .simple_config import SimpleConfig
 from .exchange_rate import FxThread
@@ -512,10 +512,11 @@ class Daemon(Logger):
                 return
             storage.decrypt(password)
         # read data, pass it to db
-        db = WalletDB(storage.read(), storage=storage, manual_upgrades=manual_upgrades)
-        if db.requires_split():
+        try:
+            db = WalletDB(storage.read(), storage=storage, manual_upgrades=manual_upgrades)
+        except WalletRequiresSplit:
             return
-        if db.requires_upgrade():
+        except WalletRequiresUpgrade:
             return
         if db.get_action():
             return