Swaps over Nostr

- Separation between SwapManager and its transport:
   Legacy transpport uses http, Nostr uses websockets
 - The transport uses a context to open/close connections.
   This context is not async, because it needs to be called
   from the GUI
 - Swapserver fees values are initialized to None instead
   of 0, so that any attempt to use them before the swap
   manager is initialized will raise an exception.
 - Remove swapserver fees disk caching (swap_pairs file)
 - Regtests use http transport
 - Android uses http transport (until QML is ready)

tests/regtest.py

@@ -79,10 +79,14 @@ class TestLightningSwapserver(TestLightning):
     agents = {
         'alice': {
             'use_gossip': 'false',
+            'swapserver_url': 'http://localhost:5455',
+            'nostr_relays': "''",
         },
         'bob': {
             'lightning_listen': 'localhost:9735',
             'enable_plugin_swapserver': 'true',
+            'swapserver_port': '5455',
+            'nostr_relays': "''",
         }
     }