distinguish kZSTDNotFinalCompression in compression string

Summary:
This confused some users who were getting compression type from the logs.
Closes https://github.com/facebook/rocksdb/pull/3153

Differential Revision: D6294964

Pulled By: ajkr

fbshipit-source-id: 3c813376d33682dc6ccafc9a78df1a2e2528985e

util/compression.h

@@ -145,8 +145,9 @@ inline std::string CompressionTypeToString(CompressionType compression_type) {
     case kXpressCompression:
       return "Xpress";
     case kZSTD:
-    case kZSTDNotFinalCompression:
       return "ZSTD";
+    case kZSTDNotFinalCompression:
+      return "ZSTDNotFinal";
     default:
       assert(false);
       return "";