FileStore,OSD: add mechanism for injecting EIO, truncating obj

This will be used in testing repair.

Signed-off-by: Samuel Just <sam.just@inktank.com>
PG: read_error should trigger a repair in _compare_scrub_objects

Signed-off-by: Samuel Just <sam.just@inktank.com>
PG: pass authoritative scrub map to _scrub

Signed-off-by: Samuel Just <sam.just@inktank.com>
PG: don't compare auth with itself

Signed-off-by: Samuel Just <sam.just@inktank.com>
PG: better _select_auth_object debugging

Signed-off-by: Samuel Just <sam.just@inktank.com>