revlog-compression: use zstd by default (if available) As see in changeset bb271ec2fbfb, zstd is 20% to 50% faster for reading and writing. Use take advantage of the new config behavior to try zstd by default, falling back to zlib is zstd is not available on that plateform. Differential Revision: https://phab.mercurial-scm.org/D10326