This re-implements compression that was lost in the WorkFile/BufFile
refactoring. This new implementation uses Zstandard rather than zlib.

Compression can be enabled with "gp_workfile_compression=on". It is
disabled by default.
Reviewed-by: NMel Kiyama <mkiyama@pivotal.io>
Reviewed-by: NYandong Yao <yyao@pivotal.io>