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ARROW-2586: [C++] Changing the type of ListBuilder's and StructBuilder's children from unique_ptr to shared_ptr so that it can support deserialization from Parquet to Arrow with arbitrary nesting

This allows the class that is responsible for deserializing to hold a shared_ptr to the concrete type without having to static_cast from the (List|Struct)Builder's getters.

This was needed for https://github.com/apache/parquet-cpp/pull/462.

Author: Joshua Storck <joshua.storck@twosigma.com>

Closes #2034 from joshuastorck/shared_ptr_in_builders and squashes the following commits:

d5aac220 <Joshua Storck> Fixing format errors
d6c6945f <Joshua Storck> Changing the type of ListBuilder's and StructBuilder's children from unique_ptr to shared_ptr so that it can support deserialization from Parquet to Arrow with arbitrary nesting. This allows the class that is responsible for deserializing to hold a shared_ptr to the concrete type without having to static_cast from the (List|Struct)Builder's getters.