make reader_at handle random reads more efficiently for FUSE

weed/util/chunk_cache/chunk_cache_in_memory.go

@@ -31,6 +31,20 @@ func (c *ChunkCacheInMemory) GetChunk(fileId string) []byte {
 	return data
 }
 
+func (c *ChunkCacheInMemory) getChunkSlice(fileId string, offset, length uint64) ([]byte, error) {
+	item := c.cache.Get(fileId)
+	if item == nil {
+		return nil, nil
+	}
+	data := item.Value().([]byte)
+	item.Extend(time.Hour)
+	wanted := min(int(length), len(data)-int(offset))
+	if wanted < 0 {
+		return nil, ErrorOutOfBounds
+	}
+	return data[offset : int(offset)+wanted], nil
+}
+
 func (c *ChunkCacheInMemory) SetChunk(fileId string, data []byte) {
 	localCopy := make([]byte, len(data))
 	copy(localCopy, data)