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ARROW-2121: [Python] Handle object arrays directly in pandas serializer.

The goal here is to get the best of both the `pandas_serialization_context` (speed at serializing pandas dataframes containing strings and other objects) and the `default_serialization_context` (correctly serializing a large class of numpy object arrays).

This PR sort of messes up the function `pa.pandas_compat.dataframe_to_serialized_dict`. Is that function just a helper function for implementing the custom pandas serializers? Or is it intended to be used in other places.

TODO in this PR (assuming you think this approach is reasonable):

- [x] remove `pandas_serialization_context`
- [x] make sure this code path is tested
- [x] double check performance/behavior

cc @wesm @pcmoritz @devin-petersohn

Author: Robert Nishihara <robertnishihara@gmail.com>
Author: Wes McKinney <wes.mckinney@twosigma.com>

Closes #1581 from robertnishihara/pandasserialization and squashes the following commits:

c551ed6c [Wes McKinney] Add benchmark for zero-copy pandas deserialization
e12cd721 [Robert Nishihara] Use highest protocol.
6e3eceb6 [Robert Nishihara] Fix bug.
31b3cbee [Robert Nishihara] Fixes.
0fc9ecfd [Robert Nishihara] Update documentation and testing.
b796fbdb [Robert Nishihara] Handle object arrays directly in pandas serializer.