Add log output during initial header sync (Jonas Schnelli)
Pull request description:
The non debug log output is completely quiet during the header sync. I see two main reasons to add infos about the state of the initial header sync...
* users may think the node did fail to start sync
* it's a little complicate to check if your getting throttled during header sync (repeatedly calling `getchaintips` or similar)
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Backport of Core PR15615