We don't add or maintain these, and they are of little value, as
well as having the effect of polluting diffs.
They are also wrong, i.e DEFAULT_SCRIPTCHECK_THREADS is not in
validation.h.
Avoid permanently storing headers from a peer, unless the headers are part of a
chain with sufficiently high work. This prevents memory attacks using low-work
headers.
Designed and co-authored with Pieter Wuille.
