unify byte order in zktrie node's fields being stored

[noel2004]

Currently the fields inside zktrie's node which being persisted in stateDb use different byteorder and may bring in many confusions:

• the left/right hash in middile node are persisted by little endian
• the nodeKey in leaf node is persisted by little endian
• other fields (value fields in leaf node) are persisted by big endian

This PR unify all the fields to big endian. A spec for the format of stored data is also added

What are broken?

This patch would break all the geth's db currently run with zktrie:true so they (if there is any) must be inited with completly new data.