Revert "Stop using the dedicated thread for the merger store compress…

…or."

This reverts commit 1c0e897.

quickwit/quickwit-indexing/src/actors/merge_executor.rs

@@ -460,10 +460,7 @@ impl MergeExecutor {
         ];
         directory_stack.extend(split_directories.into_iter());
         let union_directory = UnionDirectory::union_of(directory_stack);
-        let mut union_index = open_index(union_directory, self.doc_mapper.tokenizer_manager())?;
-        union_index
-            .settings_mut()
-            .docstore_compress_dedicated_thread = false;
+        let union_index = open_index(union_directory, self.doc_mapper.tokenizer_manager())?;
 
         ctx.record_progress();
         let _protect_guard = ctx.protect_zone();