tabletserver: list bind vars support complete

You can now specify a new syntax for queries that
have an IN clause:
select * from t where id in ::list
The corresponding bind variable would be:
{"list": field_types.List([1,2,3])}

- List bind vars are only allowed for IN and NOT IN clauses.
- A bind variable can be a list if and only if
  it's referred as ::list.
- A list must contain at least one value.

For the python client, you need to supply lists using the
field_types.List class. This is because of legacy behavior
where lists were previously encoded as strings.
We should soon change this API to use native lists once we
confirm that no one will be affected by this change.