ZSTD to use CompressionOptions.level

Summary: Now ZSTD hard code level 1. Change it to use the compression level setting.

Test Plan: Run it with hacked codes of sst_dump and show ZSTD compression sizes with different levels.

Reviewers: rven, anthony, yhchiang, kradhakrishnan, igor, IslamAbdelRahman

Reviewed By: IslamAbdelRahman

Subscribers: yoshinorim, leveldb, dhruba

Differential Revision: https://reviews.facebook.net/D52041

util/compression.h

@@ -620,7 +620,7 @@ inline bool ZSTD_Compress(const CompressionOptions& opts, const char* input,
   size_t compressBound = ZSTD_compressBound(length);
   output->resize(static_cast<size_t>(output_header_len + compressBound));
   size_t outlen = ZSTD_compress(&(*output)[output_header_len], compressBound,
-                                input, length, 1 /* level */);
+                                input, length, opts.level);
   if (outlen == 0) {
     return false;
   }