`ZlibCompressor` seems not to respect compression level

[asinghvi17]

https://github.com/JuliaIO/Zarr.jl/blob/f436713a10a9d16ca82417176e1c8f6a3e26bc8a/src/Compressors.jl#L139-L147

It looks like the clevel field in the compressor is not accessed, even in zcompress? Not sure if this is intentional or I am missing something.