Merge #11490: Disconnect from outbound peers with bad headers chains

e065249 Add unit test for outbound peer eviction (Suhas Daftuar)
5a6d00c Permit disconnection of outbound peers on bad/slow chains (Suhas Daftuar)
c60fd71 Disconnecting from bad outbound peers in IBD (Suhas Daftuar)

Pull request description:

  The first commit will disconnect an outbound peer that serves us a headers chain with insufficient work while we're in IBD.

  The second commit introduces a way to disconnect outbound peers whose chains fall out of sync with ours:

  For a given outbound peer, we check whether their best known block (which is known from the blocks they announce to us) has at least as much work as our tip.  If it doesn't, we set a 20 minute timeout, and if we still haven't heard about a block with as much work as our tip had when we set the timeout, then we send a single getheaders message, and wait 2 more minutes.  If after two minutes their best known block has insufficient work, we disconnect that peer.

  We protect 4 of our outbound peers (who provide some "good" headers chains, ie a chain with at least as much work as our tip at some point) from being subject to this logic, to prevent excessive network topology changes as a result of this algorithm, while still ensuring that we have a reasonable number of nodes not known to be on bogus chains.

  We also don't require our peers to be on the same chain as us, to prevent accidental partitioning of the network in the event of a chain split.  Note that if our peers are ever on a more work chain than our tip, then we will download and validate it, and then either reorg to it, or learn of a consensus incompatibility with that peer and disconnect.  This PR is designed to protect against peers that are on a less work chain which we may never try to download and validate.

Tree-SHA512: 2e0169a1dd8a7fb95980573ac4a201924bffdd724c19afcab5efcef076fdbe1f2cec7dc5f5d7e0a6327216f56d3828884f73642e00c8534b56ec2bb4c854a656

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