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ARROW-5380: [C++] Fix memory alignment UBSan errors.

- Add utility methods for unaligned loads use where errors
  are discovered.
- Upgrade version of flatbuffers to avoid issues with unaligned
  load in that library
- Discover bug in spec that makes zero-copy well defined behavior
  virtually impossible with flatbuffers (need to discuss on ML).  For now I'm
  not turning on ASAN and will file a follow-up JIRA to track this.

Still needed:
 - [ ] Performance testing
 - [X] Discuss flatbuffers issues (I sent e-mail to LM)

Author: Micah Kornfield <emkornfield@gmail.com>
Author: emkornfield <emkornfield@gmail.com>

Closes #4757 from emkornfield/ubsan_mem and squashes the following commits:

5528584a7 <emkornfield> remove TODO
db49fbbb4 <Micah Kornfield> Ubsan excluding flatbuffers