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ARROW-521: [C++] Track peak allocations in default memory pool

This should enable us to remove the `parquet::MemoryAllocator` implementation in parquet-cpp

Author: Wes McKinney <wes.mckinney@twosigma.com>

Closes #330 from wesm/ARROW-521 and squashes the following commits:

10531c4 [Wes McKinney] Move max_memory_ member to DefaultMemoryPool, add default virtual max_memory() to MemoryPool
a0d134d [Wes McKinney] Add max_memory() method to MemoryPool, leave implementation to subclasses