Don't send getheaders message when another request is outstanding

Change getheaders messages so that we wait up to 2 minutes for a response to a
prior getheaders message before issuing a new one.

Also change the handling of the getheaders message sent in response to a block
INV, so that we no longer use the hashstop variable (including the hash stop
will just mean that if our peer's headers chain is longer, then we won't learn
it, so there's no benefit to using hashstop).

Also, now respond to a getheaders during IBD with an empty headers message
(rather than nothing) -- this better conforms to the intent of the new logic
that it's better to not ignore a peer's getheaders message, even if you have
nothing to give. This also avoids a lot of functional tests breaking.

p2p_segwit.py is modified to use this same strategy, as the test logic (of
expecting a getheaders after a block inv) would otherwise be broken.

test/functional/feature_minchainwork.py

@@ -82,7 +82,7 @@ class MinimumChainWorkTest(BitcoinTestFramework):
         msg.hashstop = 0
         peer.send_and_ping(msg)
         time.sleep(5)
-        assert "headers" not in peer.last_message
+        assert ("headers" not in peer.last_message or len(peer.last_message["headers"].headers) == 0)
 
         self.log.info("Generating one more block")
         self.generate(self.nodes[0], 1)