Performance: diffset store

[Giulio2002]

Summary

Introduces a file-based diffset storage system (DiffsetDatabase) for storing per-block state change sets, with zero-allocation optimizations for high-performance serialization.

Key Changes

• New DiffsetDatabase (db/diffsetdb/) - File-based storage for block diffsets

  • Stores each block's diffset as a separate file: {blockNumber}+{blockHash}.diffset
  • Uses a simple mutex for thread safety
  • Async writes with goroutine to avoid blocking callers

• Zero-allocation optimizations:

  • Pre-allocated reusable buffers for serialization (serializeBuf), path building (pathBuf, filenameBuf)
  • SerializeDiffSet uses fixed-size arrays instead of maps for dictionary building (linear search beats map for small N)
  • Pre-computed hex lookup table for filename encoding
  • StateChangeSet.SerializeTo() method for buffer reuse

• Serialization improvements (db/state/changeset/):

  • Empty diffset fast path (returns 5 bytes: dict len + diffSet len)
  • Pre-calculates total buffer size to avoid reallocations
  • Uses binary.BigEndian.AppendUint* for inline encoding

Benchmark Results

Operation	Allocs	Bytes/op
WriteDiffSet	10	~1 KB
SerializeDiffSet (with buffer reuse)	0	0 B
ReadDiffSet	31	69 KB