Merge upstream Astyanax from Netflix 1.56.26 to 1.56.49.

There are only 2 major differences between the Netflix and Bazaarvoice versions of Astyanax:
*   Removes the assumption that a token is a BigInteger to better support ByteOrderedPartitioner.
*   Adds support for Thrift APIs describe_splits() and describe_splits_ex().

This merge incorporates a re-organization of the directory structure in the upstream Netflix version.  This pull request does not include CQL-based driver support added in Netflix Astyanax 2.0.x.