Advises kernel to use random access for disk bucket mmaps (anza-xyz#2140

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bucket_map/src/bucket_storage.rs

@@ -433,6 +433,10 @@ impl<O: BucketOccupied> BucketStorage<O> {
                 std::env::current_dir(),
             );
         });
+        // Access to the disk bucket files are random (excluding the linear search on collisions),
+        // so advise the kernel to treat the mmaps as such.
+        #[cfg(unix)]
+        mmap.advise(memmap2::Advice::Random).unwrap();
         measure_mmap.stop();
         stats
             .new_file_us