rados: prefetch skiplist/footer

Signed-off-by: Noah Watkins <noahwatkins@gmail.com>
rados: cache computed blocks to read

Signed-off-by: Noah Watkins <noahwatkins@gmail.com>
rados: read from pre-computed metadata

Signed-off-by: Noah Watkins <noahwatkins@gmail.com>
rados: issue vector op to each obj

Signed-off-by: Noah Watkins <noahwatkins@gmail.com>
rados: add async read error checks

Signed-off-by: Noah Watkins <noahwatkins@gmail.com>
rados: use per stripe memory contexts

Signed-off-by: Noah Watkins <noahwatkins@gmail.com>
rados: prefetch stripes up to set byte limit

Signed-off-by: Noah Watkins <noahwatkins@gmail.com>