ARROW-6307: [Java] Provide RLE vector

[liyafan82]

RLE (run length encoding) is a widely used encoding/decoding technique. Compared with other encoding/decoding techniques, it is easier to work with the encoded data.

We want to provide an RLE vector implementation in Arrow. The design details include:

1. RleVector implements ValueVector.
2. the data structure of RleVector includes an inner vector, plus a buffer storing the end indices for runs.
3. we provide random access, with time complexity O(log(n)), so it should not be used frequently.
4. In the future, we will provide iterators to access the vector in sequence.
5. RleVector does not support update, but supports appending.
6. In the future, we will provide encoder/decoder to efficiently transform encoded/decoded vectors.

[emkornfield]

I took a quick glance over this PR and I think it raises several issues that it doesn't pay to tackle until the community decides to support RLE.

First I'm pretty sure getValueCount should be the decoded value count.
Second, I'm not sure the semantics of the Transfer pair are correct. How the vector is copied probably depends on if the target vector is an RleVector or whether it is just a normal type vector.

[emkornfield]

I'm going to close this until we come to consensus on the RLE Vectors/Arrays on the mailing list.

[liyafan82]

I'm going to close this until we come to consensus on the RLE Vectors/Arrays on the mailing list.

Sure. Thanks for your effort.
There are some points that require further discussing.