Disconnecting from bad outbound peers in IBD

When in IBD, we'd like to use all our outbound peers to help us
sync the chain.  Disconnect any outbound peers whose headers have
insufficient work.

Github-Pull: #11490
Rebased-From: c60fd71a65

test/functional/minchainwork.py

@@ -27,6 +27,7 @@ class MinimumChainWorkTest(BitcoinTestFramework):
     def set_test_params(self):
         self.setup_clean_chain = True
         self.num_nodes = 3
+
         self.extra_args = [[], ["-minimumchainwork=0x65"], ["-minimumchainwork=0x65"]]
         self.node_min_work = [0, 101, 101]
 
@@ -74,6 +75,13 @@ class MinimumChainWorkTest(BitcoinTestFramework):
         self.nodes[0].generate(1)
 
         self.log.info("Verifying nodes are all synced")
+
+        # Because nodes in regtest are all manual connections (eg using
+        # addnode), node1 should not have disconnected node0. If not for that,
+        # we'd expect node1 to have disconnected node0 for serving an
+        # insufficient work chain, in which case we'd need to reconnect them to
+        # continue the test.
+
         self.sync_all()
         self.log.info("Blockcounts: %s", [n.getblockcount() for n in self.nodes])