Allow cheaply cloning a StoreReader to enable user control over block…

… cache usage.

src/store/reader.rs

@@ -112,6 +112,27 @@ impl Sum for CacheStats {
     }
 }
 
+impl Clone for StoreReader {
+    /// Creates a clone of the given store reader but using an independent block cache.
+    fn clone(&self) -> Self {
+        Self {
+            decompressor: self.decompressor,
+            data: self.data.clone(),
+            cache: BlockCache {
+                cache: self
+                    .cache
+                    .cache
+                    .as_ref()
+                    .map(|cache| Mutex::new(LruCache::new(cache.lock().unwrap().cap()))),
+                cache_hits: Default::default(),
+                cache_misses: Default::default(),
+            },
+            skip_index: Arc::clone(&self.skip_index),
+            space_usage: self.space_usage.clone(),
+        }
+    }
+}
+
 impl StoreReader {
     /// Opens a store reader
     ///