Serial fix node group

[aziz-mu]

Fixing serial in node-groups branch. Also added a test/dataset for serial copy which required more than one node group

[ray6080]

Just took a quick look. I think there are still something missing in this PR before I dive into more detailed review:

1. You only handled the case for CSV, can you also add the logic to Parquet and NPY?
2. It seems CombineChunksToBatch is not performing zero-copy internally (found something here, double check their source code? The concatenation can be costful. We should avoid it. Are there any other solutions in your mind? if not, can you try to explore my previous suggestion on using ChunkedArray? For which you need to change ValueVector to be able to keep ChunkedArray instead of Array.
3. There is no distinction between forcing order preserving or not (has SERIAL or not) in reading from csv files. This might be fine for CSV, as it has to be in a serial mode anyways, but when you replicate changes to Parquet and NPY, we do need to consider the case that we can read concurrently (which is critical) if there is no SERIAL. So better to differentiate the reading logic of the two cases regarding to preserving order or not.

[ray6080]

Usually we don't manually config num of threads for copy. Is there any special reason here?

[aziz-mu]

I wanted this test to always fail if this feature wasn't working. Since it's based on a race condition, I found that running it with 2 threads made it a lot more likely to fail than running it with default parameters. Testing with default parameters, this test failed only 5/15 times, and with 2 threads it failed 15/15 times, which is what I wanted.

Not exactly sure why more threads doesn't make it more likely to fail.

[aziz-mu]

Also, I know that there is a PARALLELISM command for the tests, but I'm not sure if that affects the actual copy command, which is the command where is issue occurs