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)

run_electrum

@@ -326,6 +326,7 @@ def main():
             'cmd': 'gui',
             SimpleConfig.GUI_NAME.key(): 'qml',
             SimpleConfig.WALLET_USE_SINGLE_PASSWORD.key(): True,
+            SimpleConfig.SWAPSERVER_URL: 'https://swaps.electrum.org/api',
         }
         if util.get_android_package_name() == "org.electrum.testnet.electrum":
             # ~hack for easier testnet builds. pkgname subject to change.