perf+sec: 15 improvements across consensus, DB, network, sync

CONSENSUS SECURITY (main.cpp):
- Re-enable PoS kernel verification post-IBD (was unconditionally disabled)
- Re-enable coinstake reward validation post-IBD (was commented out)
- Re-enable anti-spam difficulty check (was if(false && ...))

SYNC PERFORMANCE (main.cpp):
- Batch address index writes in ConnectBlock (hundreds of DB ops → one per address)
- Throttle IBD printfs (per-block → per-10K-blocks or fDebug-gated)

DATABASE (txdb-rocksdb.cpp/h, txdb-base.cpp):
- Non-batched WriteRaw: WAL sync=false (was fsync per write)
- UTXO cache: FIFO eviction → true LRU with access-order tracking
- RocksDB memtable: 64MB → 256MB + max_write_buffer_number=4
- pendingBatch: std::map → std::unordered_map (O(log n) → O(1))
- max_open_files: 1000 → unlimited

NETWORK (net.cpp, netbase.cpp):
- TCP_NODELAY on all sockets (disable Nagle's algorithm)
- SO_KEEPALIVE on all sockets (faster dead-peer detection)
- Adaptive MilliSleep: 1ms during IBD, 10ms otherwise
- writev() scatter-gather I/O for send() coalescing (up to 16 msgs/syscall)
- O(1) CountInFlight counter (was O(n) scan of entire header map)

src/syncmanager.cpp

@@ -44,6 +44,11 @@ static const int HEADER_FRONT_MAX_AHEAD = 32768;
 std::map<uint256, CSyncManager::HeaderNode> mapHeaders;
 uint256 hashBestHeader = 0;
 int64_t nLastNewHeaderTime = 0;
+
+// O(1) in-flight counter — replaces the O(n) scan in CountInFlight().
+// Incremented when fRequested transitions false→true; decremented when an
+// entry with fRequested==true is erased from mapHeaders.
+static size_t g_nInFlight = 0;
 }
 
 CSyncManager g_syncManager;
@@ -151,6 +156,8 @@ void CSyncManager::PruneHeaders()
             if (it->second.nHeight > nProtectFloor &&
                 nNow - it->second.nInsertTime >= HEADER_SYNC_TTL_MICROS)
             {
+                if (it->second.fRequested)
+                    --g_nInFlight;
                 it = mapHeaders.erase(it);
                 ++nEvicted;
             }
@@ -188,7 +195,11 @@ void CSyncManager::PruneHeaders()
         const size_t nTarget = (size_t)MAX_HEADER_SYNC_CACHE * 3 / 4;
         size_t i = 0;
         while (mapHeaders.size() > nTarget && i < vEvictable.size())
+        {
+            if (vEvictable[i]->second.fRequested)
+                --g_nInFlight;
             mapHeaders.erase(vEvictable[i++]);
+        }
 
         RecomputeBestHeader();
     }
@@ -287,14 +298,7 @@ bool CSyncManager::PathReachesChain(const std::vector<uint256>& vPath) const
 
 unsigned int CSyncManager::CountInFlight() const
 {
-    const int64_t nNow = GetTime() * 1000000;
-    unsigned int nInFlight = 0;
-    for (std::map<uint256, HeaderNode>::const_iterator it = mapHeaders.begin(); it != mapHeaders.end(); ++it)
-    {
-        if (it->second.fRequested && nNow - it->second.nLastRequestTime < HEADER_REQUEST_TIMEOUT_MICROS)
-            ++nInFlight;
-    }
-    return nInFlight;
+    return (unsigned int)g_nInFlight;
 }
 
 unsigned int CSyncManager::GetPlannerDepth() const
@@ -331,6 +335,8 @@ void CSyncManager::BlockAccepted(const uint256& hashBlock)
     if (mi == mapHeaders.end())
         return;
 
+    if (mi->second.fRequested)
+        --g_nInFlight;
     mapHeaders.erase(mi);
     if (hashBestHeader == hashBlock)
         RecomputeBestHeader();
@@ -602,7 +608,10 @@ unsigned int CSyncManager::QueueBlocksParallel(unsigned int nWindow)
         if (!mi->second.fRequested || nNow - mi->second.nLastRequestTime >= HEADER_REQUEST_TIMEOUT_MICROS)
         {
             if (!mi->second.fRequested)
+            {
                 mi->second.nFirstRequestTime = nNow;
+                ++g_nInFlight;
+            }
             mi->second.fRequested = true;
             mi->second.nLastRequestTime = nNow;
         }