storage: fix some madness about get_data_ref() and put() interacting badly

previously load_transactions() had to be called before upgrade();
now we reverse this order.

to reproduce/illustrate issue, before this commit:

try running convert_version_17 and convert_version_18
(e.g. see testcase test_upgrade_from_client_2_9_3_old_seeded_with_realistic_history)
and then in qt console:
>> wallet.storage.db.get_data_ref('spent_outpoints') == wallet.storage.db.spent_outpoints
False
>> wallet.storage.db.get_data_ref('verified_tx3') == wallet.storage.db.verified_tx
False

electrum/storage.py

@@ -226,6 +226,9 @@ class WalletStorage(Logger):
             raise Exception("storage not yet decrypted!")
         return self.db.requires_upgrade()
 
+    def is_ready_to_be_used_by_wallet(self):
+        return not self.requires_upgrade() and self.db._called_after_upgrade_tasks
+
     def upgrade(self):
         self.db.upgrade()
         self.write()
@@ -240,6 +243,7 @@ class WalletStorage(Logger):
             path = self.path + '.' + data['suffix']
             storage = WalletStorage(path)
             storage.db.data = data
+            storage.db._called_after_upgrade_tasks = False
             storage.db.upgrade()
             storage.write()
             out.append(path)