coins: use the chain's BIP173 name instead of a 'unit of account'

Updates the unit of account to be the chain_id, which is the BIP173 name
of the chain that the coins moved on.

Suggested-By: @rustyrussell

tests/utils.py

@@ -42,7 +42,7 @@ def check_coin_moves(n, account_id, expected_moves):
         assert mv['debit'] == "{}msat".format(exp['debit'])
         assert mv['tag'] == exp['tag']
         assert mv['timestamp'] > 0
-        assert mv['unit_of_account'] == 'btc'
+        assert mv['coin_type'] == 'bcrt'
         # chain moves should have blockheights
         if mv['type'] == 'chain_mvt':
             assert mv['blockheight'] is not None