Support Multiple Tuple Inserts

[robertclaus]

Added support for multiple tuple insert.

Previously

INSERT INTO <table> VALUES (<values for tuple 1>),(<values for tuple 2>);

would not work. With this change it will insert both tuples into <table>.

[zuyu]

IMHO, this PR has a major design issue where for multi-tuple insertions, we create a bunch of InsertOperator and SaveBlocksOperator, each for tuple.

[hbdeshmukh]

I agree with @zuyu. We should reconsider the design.

[jianqiao]

It may just be fine before ExecutionGenerator, I think the fix is to revise ExecutionGenerator::convertInsertTuple() and do some modifications inside InsertOperator.