Reduce getheaders spam by serializing getheader requests per peer

Introduces a counter for getheader requests that have been sent to
a peer but are pending response, reducing the number of parallel
requests a node pushes out to its peers when needing to sync large
amounts of headers. All getheader requests are serialized during
initial sync, except when a non-connecting header is received,
allowing the node to resolve issues with peers sending faulty
blocks using the DoS mechanism, and when we get an inv for a block
that we do not know, because it's possible we're only connected to
legacy nodes that do not implement header announcement properly.

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