SRC_SIZE || BLK_SIZE || HASH0 || HASH1 || ...

SRC_SIZE contains size of the source, when it was initially read.
BLK_SIZE is the blocksize used. Both are 64-bit big-endian unsigned
integers. If either size or blocksize differs, then syncer will deny
using that statefile as a precaution. HASHx is BLAKE3-256 hash output.