exchange_rate: more robust spot price against temporary network issues

We poll the fx rate provider every 2.5 minutes (unchanged).
Previously if there was any error during a tick, there was no fx rate
available in the client until the next tick.
Now, instead, we keep the last rates received with a 10 minute expiry.

One potential drawback is that previously there was instant feedback
to the user when e.g. changing proxy settings, and this is no longer
the case. E.g. consider a provider that bans Tor exit nodes. If a user
enables using a Tor proxy in the network settings, the fxrate used to
disappear immediately - but now the cached rate would still be
available.

electrum/tests/test_wallet.py

@@ -89,7 +89,8 @@ class TestWalletStorage(WalletTestCase):
 class FakeExchange(ExchangeBase):
     def __init__(self, rate):
         super().__init__(lambda self: None, lambda self: None)
-        self.quotes = {'TEST': rate}
+        self._quotes = {'TEST': rate}
+        self._quotes_timestamp = float("inf")  # spot price from the far future never becomes stale :P
 
 class FakeFxThread:
     def __init__(self, exchange):