Log block header in net_processing

Previously ChainstateManager::AcceptBlockHeader would log when it
saw a new header. This commit moves logging to the call site(s) in
net_processing. The next commits will then log which peer sent it
and whether it was part of a compact block.

This commit changes behavior:
- when multiple headers are received in a single message, only the
  last one is logged
- if any of the headers are invalid, the valid ones are not logged

This happens because net_processing calls ProcessNewBlockHeaders
with multiple headers, which then calls AcceptBlockHeader one
header at a time.

Additionally:
- when the header is received via a compact block, there's no more
  duplicate log (a later commit also unifies logging code paths)

src/validation.cpp

@@ -4403,23 +4403,6 @@ bool ChainstateManager::AcceptBlockHeader(const CBlockHeader& block, BlockValida
     if (ppindex)
         *ppindex = pindex;
 
-    // Since this is the earliest point at which we have determined that a
-    // header is both new and valid, log here.
-    //
-    // These messages are valuable for detecting potential selfish mining behavior;
-    // if multiple displacing headers are seen near simultaneously across many
-    // nodes in the network, this might be an indication of selfish mining. Having
-    // this log by default when not in IBD ensures broad availability of this data
-    // in case investigation is merited.
-    const auto msg = strprintf(
-        "Saw new header hash=%s height=%d", hash.ToString(), pindex->nHeight);
-
-    if (IsInitialBlockDownload()) {
-        LogPrintLevel(BCLog::VALIDATION, BCLog::Level::Debug, "%s\n", msg);
-    } else {
-        LogPrintf("%s\n", msg);
-    }
-
     return true;
 }