feat: finalize changeset file format

[yihuang]

• The changeset file format should be usable by versiondb directly.
• It should support a range of versions in the same file to avoid too many small files.
• It should support streaming reading/writing.
• Secondly indexed can be created separately.
• Detect and recover file corruption happens at the end.

version: int32
size: int32  # the total size of kv-pairs, so we can skip versions relatively fast.
kv-pairs: length prefixed proto msg

class StoreKVPair(ProtoEntity):
  delete = Field('bool', 1)
  key = Field('bytes', 2)
  value = Field('bytes', 3)