Add opt-in bulk block streaming (-bulkblocksync)

A single getblockstrm request makes a peer stream a contiguous range of old
blocks back-to-back as ordinary BLOCK messages, amortizing the per-block
round-trip over the whole range instead of the MAX_BLOCKS_IN_TRANSIT_PER_PEER
window. This targets the bandwidth-delay-product ceiling that dominates IBD
from few/high-latency peers below the checkpoint.

Design (off by default; negotiated via a NODE_BULKBLOCKS service bit; the
default getdata IBD path is untouched when disabled):
- protocol: NODE_BULKBLOCKS service bit + getblockstrm/blockstream messages.
- requester: in SendMessages, after FindNextBlocksToDownload, when the first
  needed block is >= BULK_TIP_MARGIN (5000) below the network tip and the peer
  advertises the bit and we are in IBD, request a contiguous range (<=128
  blocks) instead of per-block getdata; mark the range in-flight.
- server: stream the range (caps 128 blocks / 8 MiB; reads outside cs_main;
  per-peer flood throttle), then a trailing blockstream header with the actual
  count sent. Self-suppresses while the server itself is in IBD.
- received blocks ride the existing BLOCK -> ProcessNewBlock path (fully
  validated; checkpoints below 2.84M still apply); the trailing header
  reconciles partial deliveries and the range is freed on a 90s timeout, so a
  partial/withheld/refused batch falls back to the normal path (no leak, no
  permanent gap, no disconnect). In-flight tracking is by literal hash, so a
  reorg cannot orphan range entries.

Hardened against the issues found in two adversarial review passes (drain vs
timeout, partial reconciliation, ownership-guarded frees, one-shot header,
reorg-proof helpers, cs_main hold). Validated end-to-end between two local
v1.0.3 nodes (128/128 and partial serves; height advanced; no errors).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/protocol.h

@@ -285,6 +285,10 @@ extern const char* GETNSPV;
 extern const char* NSPV;
 extern const char* ALERT;
 extern const char* REJECT;
+/** Opt-in bulk block streaming (DragonX): request a contiguous range of old blocks. */
+extern const char* GETBLOCKSTREAM;
+/** Opt-in bulk block streaming (DragonX): control header preceding a streamed block range. */
+extern const char* BLOCKSTREAM;
 }; // namespace NetMsgType
 
 /* Get a vector of all valid message types (see above) */
@@ -304,6 +308,9 @@ enum ServiceFlags : uint64_t {
     NODE_NSPV = (1 << 30),
     NODE_ADDRINDEX = (1 << 29),
     NODE_SPENTINDEX = (1 << 28),
+    // Opt-in bulk block streaming (DragonX). Unauthenticated advertisement; serve/request
+    // handlers validate every block regardless, so robustness against false advertisement holds.
+    NODE_BULKBLOCKS = (1 << 27),
 
     // Bits 24-31 are reserved for temporary experiments. Just pick a bit that
     // isn't getting used, or one not being used much, and notify the