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ARROW-6866: [Java] Improve the performance of calculating hash code for struct vector

Improve the performance of hashCode(int) method for StructVector:
1. We can get the child vectors directly, so there is no need to get the name from the child vector and then use the name to get the vector.
2. The child vectors cannot be null, so there is no need to check it.

The performance improvement depends on the complexity of the hash algorithm. For computational intensive hash algorithms, the improvement can be small; while for simple hash algorithms, the improvement can be notable.

Closes #5633 from liyafan82/fly_1012_strhash and squashes the following commits:

d4a2c3b94 <liyafan82>  Improve the performance of calculating hash code for struct vector

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