[feature] Maintain maphash fork to avoid initSeed in Sum64

[gaocegege]

We do not want to initSeed in Sum64.

// Sum64 returns h's current 64-bit value, which depends on
// h's seed and the sequence of bytes added to h since the
// last call to Reset or SetSeed.
//
// All bits of the Sum64 result are close to uniformly and
// independently distributed, so it can be safely reduced
// by using bit masking, shifting, or modular arithmetic.
func (h *Hash) Sum64() uint64 {
	h.initSeed()
	return rthash(h.buf[:h.n], h.state.s)
}

[gaocegege]

We need to use consistent hash