Merge bitcoin/bitcoin#24178: p2p: Respond to getheaders if we have sufficient chainwork

a35f963edf1a14ee572abea106fb0147575e4694 Add test for getheaders behavior (Suhas Daftuar)
ef6dbe6863d92710fd2da7781e5b2aac87578751 Respond to getheaders if we have sufficient chainwork (Suhas Daftuar)

Pull request description:

  Previously, we would check to see if we were in IBD and ignore getheaders requests accordingly. However, the IBD criteria -- an optimization mostly targeted at behavior when we have peers serving us many blocks we need to download -- is difficult to reason about in edge-case scenarios, such as if the network were to go a long time without any blocks found and nodes are getting restarted during that time.

  To make things simpler to reason about, just use `nMinimumChainWork` as our anti-DoS threshold for responding to a getheaders request; as long as our chain has that much work, it should be fine to respond to a peer asking for our headers (and this should allow such a peer to request blocks from us if needed).

ACKs for top commit:
  klementtan:
    crACK a35f963edf1a14ee572abea106fb0147575e4694
  naumenkogs:
    ACK a35f963edf1a14ee572abea106fb0147575e4694
  MarcoFalke:
    review ACK a35f963edf1a14ee572abea106fb0147575e4694 🗯

Tree-SHA512: 131e3872e7fe80382ea9c1ec202d6c2dc59c006355c69000aa3f4ce6bccd02a6c689c8cb8f3542b5d9bc48bfa61edcbd1a78535c0b79018971d02bed2655d284

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