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ARROW-4635: [Java] allocateNew to use last capacity

Modified allocateNew to allocate buffers as per the last capacity. This fixes a regression that was
causing oversized allocations with StructVectors.

Author: Pindikura Ravindra <ravindra@dremio.com>

Closes #3717 from pravindra/reallocs and squashes the following commits:

dd34f591 <Pindikura Ravindra> ARROW-4635:  allocateNew to use last capacity