update to zstd1.4.8 (#610)

Zstd introduces performance improvement since 1.4.7. 
We need to benchmark and upgrade to the latest version.
Signed-off-by: Zhenhan Gong <zhenhan.gong@gmail.com>

librocksdb_sys/Cargo.toml

@@ -12,7 +12,7 @@ librocksdb_cloud_sys = { path = "librocksdb_cloud_sys" }
 libtitan_sys = { path = "libtitan_sys" }
 libz-sys = { version = "1.1", features = ["static"] }
 openssl-sys = { version = "0.9.54", optional = true, features = ["vendored"] }
-zstd-sys = "1.4.15+zstd.1.4.4"
+zstd-sys = "1.4.19+zstd.1.4.8"
 lz4-sys = "1.9"
 
 [dev-dependencies]