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/txdb-rocksdb.cpp

@@ -32,6 +32,15 @@ namespace fs = std::filesystem;
 // the same way the LevelDB backend shares its txdb singleton.
 static rocksdb::DB* g_rocksdb = nullptr;
 
+// Non-batched writes bypass WAL fsync. The TxnCommit path handles durability;
+// crash recovery replays from block files anyway. Default WriteOptions may
+// vary across RocksDB versions, so we pin sync=false explicitly.
+static const rocksdb::WriteOptions g_fastWriteOpts = []{
+    rocksdb::WriteOptions wo;
+    wo.sync = false;
+    return wo;
+}();
+
 namespace {
 
 // rocksdb::DB::Open shipped a raw DB** overload for years; newer releases
@@ -71,8 +80,9 @@ static rocksdb::Options GetRocksOptions()
     rocksdb::Options opts;
     opts.create_if_missing = false;
     opts.compression = rocksdb::kSnappyCompression;
-    opts.max_open_files = 1000;
-    opts.write_buffer_size = 64 * 1048576;
+    opts.max_open_files = -1;
+    opts.write_buffer_size = 256 * 1048576;
+    opts.max_write_buffer_number = 4;
     opts.IncreaseParallelism();             // Multi-threaded compaction.
     opts.OptimizeLevelStyleCompaction();    // Sensible defaults for a LSM workload.
 
@@ -263,7 +273,7 @@ bool CRocksTxDB::WriteRaw(const std::string& key, const std::string& value)
         pendingBatch[key] = value;
         return true;
     }
-    rocksdb::Status status = pdb->Put(rocksdb::WriteOptions(), key, value);
+    rocksdb::Status status = pdb->Put(g_fastWriteOpts, key, value);
     if (!status.ok()) {
         printf("RocksDB write failure: %s\n", status.ToString().c_str());
         return false;