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ARROW-4284: [C#] File / Stream serialization fails due to type mismatch / missing footer

- refactored ArrowBuffer API to reflect spec (immutable, value-semantics)
- refactored memory allocation API (moved padding responsibility to buffer, removed DefaultMemoryPool)
- fixed bug where reallocated memory didn't have padding zeroed
- fixed serialization bug due to faulty buffer growth function
- fixed brittle array data type visitor pattern implementation
- added unit tests for buffers and buffer builder API

Author: Christopher Hutchinson <cshutchinson@fzcorp.com>

Closes #3462 from chutchinson/bug/4284-stream-serialization-failure and squashes the following commits:

fb2cc623a <Christopher Hutchinson>  File / Stream serialization fails due to type mismatch / missing footer