implement AsyncSeek for AsyncIndexReader

Requires updating the AsyncRead implementation to cope with byte-wise
seeks to intra-chunk positions.

Uses chunk_from_offset to get locations within chunks, but tries to
avoid it for sequential read to not reduce performance from before.

AsyncSeek needs to use the temporary seek_to_pos to avoid changing the
position in case an invalid seek is given and it needs to error in
poll_complete.

Signed-off-by: Stefan Reiter <s.reiter@proxmox.com>

src/backup/index.rs

@@ -1,6 +1,7 @@
 use std::collections::HashMap;
 use std::ops::Range;
 
+#[derive(Clone)]
 pub struct ChunkReadInfo {
     pub range: Range<u64>,
     pub digest: [u8; 32],