move the parallel-compression setting to binary-cache-store, the setting

can be done now from the url e.g s3://nix-cache?parallel-compression=1
instead of nix.conf.

src/libstore/binary-cache-store.cc

@@ -149,7 +149,7 @@ void BinaryCacheStore::addToStore(const ValidPathInfo & info, const ref<std::str
     /* Compress the NAR. */
     narInfo->compression = compression;
     auto now1 = std::chrono::steady_clock::now();
-    auto narCompressed = compress(compression, *nar, settings.parallelCompression);
+    auto narCompressed = compress(compression, *nar, parallelCompression);
     auto now2 = std::chrono::steady_clock::now();
     narInfo->fileHash = hashString(htSHA256, *narCompressed);
     narInfo->fileSize = narCompressed->size();

src/libstore/binary-cache-store.hh

@@ -19,6 +19,8 @@ public:
     const Setting<bool> writeNARListing{this, false, "write-nar-listing", "whether to write a JSON file listing the files in each NAR"};
     const Setting<Path> secretKeyFile{this, "", "secret-key", "path to secret key used to sign the binary cache"};
     const Setting<Path> localNarCache{this, "", "local-nar-cache", "path to a local cache of NARs"};
+    const Setting<bool> parallelCompression{this, false, "parallel-compression",
+        "enable multi-threading compression, available for xz only currently"};
 
 private:
 

src/libstore/globals.hh

@@ -174,9 +174,6 @@ public:
         "Whether to compress logs.",
         {"build-compress-log"}};
 
-    Setting<bool> parallelCompression{this, false, "parallel-compression",
-        "Whether to enable parallel compression, only supported with xz currently"};
-
     Setting<unsigned long> maxLogSize{this, 0, "max-build-log-size",
         "Maximum number of bytes a builder can write to stdout/stderr "
         "before being killed (0 means no limit).",