Implemented as a seperate struct SeekableCachedChunkReader that contains
the original as an Arc, since the read_at future captures the
CachedChunkReader, which would otherwise not work with the lifetimes
required by AsyncRead. This is also the reason we cannot use a shared
read buffer and have to allocate a new one for every read. It also means
that the struct items required for AsyncRead/Seek do not need to be
included in a regular CachedChunkReader.
This is intended as a replacement for AsyncIndexReader, so we have less
code duplication and can utilize the LRU cache there too (even though
actual request concurrency is not supported in these traits).
Signed-off-by: Stefan Reiter <s.reiter@proxmox.com>