feat(bitcore-node): Implement multithreaded sync for UTXO chains (#3600)

[bittoby]

Description

Resolves #3600

UTXO chain sync is currently bottlenecked by strictly sequential block fetching — each block must be fully fetched and processed before the next is even requested. This PR parallelizes the fetch step while keeping the store step sequential (required for UTXO ordering, nextBlockHash pointers, and height-from-parent validation).

The implementation follows the existing EVM multi-thread sync pattern (evm/p2p/sync.ts, evm/p2p/syncWorker.ts, evm/p2p/p2p.ts) with two complementary approaches:

Phase 1 — Sliding-window prefetch (single-thread mode):
Prefetches up to prefetchSize blocks ahead via P2P while the current block is being processed. Used for near-tip sync and post-initial-sync catch-up. Configurable via prefetchSize config option (default: 10, set to 0 to disable).

Phase 2 — Worker thread sync (multi-thread mode):
Worker threads fetch raw blocks via Bitcoin Core RPC (getblock <hash> 0), while the coordinator buffers out-of-order responses and processes them in strict height order. Used during initial sync only, then automatically switches to single-thread P2P mode. Configurable via threads config option (default: CPU cores - 1, set to 0 to disable).

Dual-mode pattern matches EVM: the in-memory initialSyncComplete flag is checked before the DB read to avoid overwriting the flag set by the INITIALSYNCDONE event.

Changelog

• Add UtxoMultiThreadSync coordinator class that manages worker threads, distributes block hashes, and processes blocks in height order using an ordered buffer (src/modules/bitcoin/sync.ts)
• Add UtxoSyncWorker worker thread that fetches raw block hex via Bitcoin Core RPC using plain http/https — no external dependencies in worker context (src/modules/bitcoin/syncWorker.ts)
• Refactor BitcoinP2PWorker.sync() to support dual-mode sync: multi-thread during initial sync, single-thread with sliding-window prefetch near tip (src/modules/bitcoin/p2p.ts)
• Extract getHeadersForSync() and handleGenesisBlock() as shared helper methods in BitcoinP2PWorker
• Add useMultiThread() method matching the existing EVM pattern
• Add prefetchSize and threads config options to IUtxoNetworkConfig (src/types/Config.ts)
• Add 13 unit tests covering prefetch behavior, sequential processing order, dual-mode switching, ordered block processing, thread management, and INITIALSYNCDONE emission

Testing Notes

• All 238 unit tests pass (npm run test:unit), including 13 new tests
• TypeScript compiles cleanly under strict mode
• ESLint passes with zero warnings
• No new external dependencies added — worker threads use Node.js built-in http/https for RPC calls
• The worker thread path resolves correctly in compiled output (build/src/modules/bitcoin/syncWorker.js via __dirname)
• Integration tests are unaffected — UtxoMultiThreadSync constructor is inert (no threads spawned until sync() is called), and stop() is safe when called before sync()
• To test configuration, add to bitcore.config.json under a UTXO chain network:

  {
    "threads": 4,
    "prefetchSize": 10
  }
  

Set threads: 0 to disable multi-thread sync entirely. Set prefetchSize: 0 to disable prefetching in single-thread mode.

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Checklist

• I have read CONTRIBUTING.md and verified that this PR follows the guidelines and requirements outlined in it.

[bittoby]

@kajoseph could you please review this PR? Welcome to any feedback.
thank you