[Memory Engine] add WTDMemtable for WriteTxnData

[vagetablechicken]

Ref #3469 (comment)

1. MemTable is splited into two types, one for Rowset(the origin), one for WriteTxnData.
2. DeltaWriter doesn't have to add func write(PartialRow), so let DeltaWriter write const void*. RSMemTable will convert it to Tuple*, and WTDMemTable will convert it to PartialRow.
3. Add UT for writing & reading WTD file.