partial-writes using jsonpatch

- partial writes are append only.

 - StoredDict objects will append partial writes to the wallet
   file when items are added, replaced, removed.

 - Lists in the wallet file that have not been registered
   as StoredObject are converted to StoredList, which
   overloads append() and remove(). Those methods too will
   append partial writes to the wallet file.

 - Unlike the old jsonpatch branch, this branch does not support
   file encryption. Encrypted files always fully rewritten, even
   if the change before encryption is a partial write.

contrib/requirements/requirements.txt

@@ -7,6 +7,7 @@ aiohttp_socks>=0.3
 certifi
 bitstring
 attrs>=20.1.0
+jsonpatch
 
 # Note that we also need the dnspython[DNSSEC] extra which pulls in cryptography,
 # but as that is not pure-python it cannot be listed in this file!