Make Zstandard compression level a configurable option
I've played with using the different compression algorithms on the RGWs and the default compression level for Zstandard (level 5) seems too high. I feel like the default should actually be level 1, which makes it almost as fast as snappy/lz4 but at zlib compression ratios. Since some people may want to achieve higher compression ratios at the cost of CPU time, could we make it a configurable option? Zstandard compression levels can be between negative 5 and 22.