Complete SERIAL data type support

[ray6080]

SERIAL is introduce in #1493 but not completely supported. The following are what is missing currently:

• [x] RETURN SERIAL values. Add SERIAL to Value and FlatTupleIterator.
• [ ] Expression evaluations over SERIAL. We should cast SERIAL to INT64 for expression evaluations.
• [ ] Updates. (Deletions should be handled automatically, no update is allowed on serial values, insertions need to be double checked)
• [ ] Allow SERIAL as non primary key properties in node table.
• [x] Index lookup over SERIAL properties.
• [x] Add SERIAL in apis (not prioritized).