Add support for zstd-compressed corpora

[Daniel Mitterdorfer]

Rally supports various compression formats such as gz or bzip. It does not support the zstd format which is perfoming significantly better in disk usage and decompression speed in my experiments. I've compressed 183GB corpus with pbzip2 and pzstd, both with the maximum compression level that is supported by the respective tool.

Format	Size on disk [GB]	Size on disk [GB]	Relative size [%]
bzip	18613471805	18	100
zstd	11215205385	11	60

Also decompression speed is vastly superior (times measured with time, table contains the output of real, i.e. wall clock time):

Format	Time to decompress [s]	Relative time [%]
bzip	388	100
zstd	144	36

Therefore I propose to add support for zstd compression to Rally similar to bzip support: The fast option would require pzstd to be on PATH and a fallback can be based on the Python zstd implementation.

For reference:

• Compress data: pzstd -19 corpus.json -o corpus.json.zstd (19 denotes the maximum compression level)
• Decompress data: pzstd -d corpus.json.zstd -o corpus.json