interface: faster chain resolution: add headers_cache

We try to predict the next headers the interface will ask for,
and request them ahead of time, to be kept in the headers_cache.

This saves network latency/round-trips, for a bit more memory usage
and in some cases for more bandwidth.
Note that due to PaddedRSTransport.WAIT_FOR_BUFFER_GROWTH_SECONDS,
latency saved here can be longer than "real" network latency.

This speeds up
- binary search greatly,
- backwards search to a small degree
  (although not that much as its algorithm should be changed a bit to make it cache-friendly)
- catch-up greatly, if it's <10 blocks behind

What remains is to speed up catch-up in case we are behind by many thousands of block.
That behaviour is left unchanged here. The issue there is that we request chunks sequentially.
So e.g. 1 chunk (2016 blocks) per 1 second.

tests/test_network.py

@@ -46,6 +46,9 @@ class MockInterface(Interface):
     async def run(self):
         return
 
+    async def _maybe_warm_headers_cache(self, *args, **kwargs):
+        return
+
 
 class TestNetwork(ElectrumTestCase):