[feature] [tokudb] Issue: #63 Add compression ratio statistics per FT

TokuDB has status to expose the compression ratio:

    mysql> show global status like 'tokudb%COMPRESSION_RATIO';

    +---------------------------------------+----------+

    | Variable_name                         | Value    |

    +---------------------------------------+----------+

    | Tokudb_LEAF_NODE_COMPRESSION_RATIO    | 5.850260 |

    | Tokudb_NONLEAF_NODE_COMPRESSION_RATIO | 0.564453 |

    | Tokudb_OVERALL_NODE_COMPRESSION_RATIO | 4.528809 |

    +---------------------------------------+----------+

but this a global level ratio, and sometimes the users may be interested

in table level compression ratio.

This patch address this request by exposing FT level compresstion ratio

through information_schema.TokuDB_fractal_tree_info table. We add two

columns, leaf_compress_ratio and internal_compress_ratio, which indicate

the compression ratio of leaf nodes and internal nodes accordingly.

Limitations, the comppression ratio infos are maintained in memeory, and

will lost when server is restarted or ft get evicted from memory.

Maybe in future there infos will be persisted in ft file, when this feature

is accepted by PerconaFT upstream.