storage: encapsulate type conversions of stored objects using

decorators (instead of overloading JsonDB._convert_dict and
 _convert_value)
 - stored_in for elements of a StoreDict
 - stored_as for singletons
 - extra register methods are defined for key conversions

This commit was adapted from the jsonpatch branch

electrum/submarine_swaps.py

@@ -19,7 +19,7 @@ from .lnutil import REDEEM_AFTER_DOUBLE_SPENT_DELAY, ln_dummy_address
 from .bitcoin import dust_threshold
 from .logging import Logger
 from .lnutil import hex_to_bytes
-from .json_db import StoredObject
+from .json_db import StoredObject, stored_in
 from . import constants
 from .address_synchronizer import TX_HEIGHT_LOCAL
 from .i18n import _
@@ -87,6 +87,7 @@ class SwapServerError(Exception):
         return _("The swap server errored or is unreachable.")
 
 
+@stored_in('submarine_swaps')
 @attr.s
 class SwapData(StoredObject):
     is_reverse = attr.ib(type=bool)