[JAVA] Reduce Heap Usage (Phase 3): consolidate buffers

[asfimport]

Consolidate buffers for reducing the volume of objects and heap usage

<validity + data> => single buffer for fixed width
< validity + offsets> = single buffer for var width, list vector

Reporter: Siddharth Teotia / @siddharthteotia
Assignee: Pindikura Ravindra / @pravindra

Related issues:

• [JAVA] Reduce Heap Usage per Vector (blocks)
• [Java] Benchmark results for ARROW-1807 (is related to)

PRs and other links:

• GitHub Pull Request #3121

_{Note: This issue was originally created as ARROW-1807. Please see the migration documentation for further details.}