txnstorage: add Row.Value
txnstorage: remove Table.Index, rename IndexEntries to Index
txnstorage: merge PhysicalRow, MVCC, MVCCValues to PhysicalRow, Version
txnstorage: remove Row type param from Iter and TableIter
txnstorage: add Version.ID; fix Table.Iter
txnstorage: use btree path hinting
txnstorage, txnengine: add ID type
txnstorage: use slice instead of map in DataValue
txnstorage: add table benchmark
txnstorage: use int64 ID
txnstorage: optimize locking of Table
txnstorage: fix log tail
txnengine: add Request and Response enum
txnengine: remove ErrExisted
txnengine: remove ErrDatabaseNotFound
txnengine: remove ErrRelationNotFound
txnengine: remove ErrDefNotFound
txnengine: remove ErrIterNotFound
txnengine: remove ErrColumnNotFound, ErrReadOnly
txnengine: remove error reflection in DoTxnRequest

Approved by: @yingfeng, @zhangxu19830126