add AsyncIndexReader

implements AsyncRead as well as Stream for an IndexFile and a store
that implements AsyncReadChunk

we can use this to asyncread or stream the content of a FixedIndex or
DynamicIndex

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>

src/backup.rs

@@ -200,3 +200,6 @@ pub use datastore::*;
 
 mod catalog_shell;
 pub use catalog_shell::*;
+
+mod async_index_reader;
+pub use async_index_reader::*;