Vectorization : Support loading bucketed tables using sorted dynamic partition optimizer - II

Followup to HIVE-20510 with remaining issues,

1. Avoid using Reflection.
2. In VectorizationContext, use correct place to setup the VectorExpression. It may be missed in certain cases.
3. In BucketNumExpression, make sure that a value is not overwritten before it is processed. Use a flag to achieve this.

cc gopalv