arrow

ARROW-6352: [Java] Add implementation of DenseUnionVector

Today only Sparse unions are supported.  We should have a dense union implementation vector that conforms to the IPC protocol (the current sparse union vector doesn't do this and there are other JIRAs covering making it compatible).

Closes #5473 from liyafan82/fly_0923_union and squashes the following commits:

47b31e778 <liyafan82>  Resolve comments
d95288039 <liyafan82>  Support multiple vectors for each vector type
95c8afb6f <liyafan82>  Support isNull with validity buffer of underlying vectors
fdf9f3a65 <liyafan82>  Support type id with byte
86e92dc6d <liyafan82>  Support arbitrary relative type
1d52a6aac <liyafan82>  Replace release method with close method
afd919925 <liyafan82>  Separate implementations for UnionVector and DenseUnionVector
b1cf2bbe7 <liyafan82>  Add implementation of DenseUnionVector

Authored-by: liyafan82 <fan_li_ya@foxmail.com>
Signed-off-by: Micah Kornfield <emkornfield@gmail.com>