Respond to getheaders if we have sufficient chainwork

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 getting
restarted during that time.

To make things simpler to reason about, just use nMinimumChainWork as our
anti-DoS threshold; 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).

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