PG: make _select_auth_object smarter

Previously, we just picked the first one to have the object in
question.  Now, we will attempt to choose one that has as
much of the following as possible:
1) has the object (there must be one)
2) has an object_info attr
3) has a valid object_info attr
4) has an object_info whose size matches the scrubbed size

Signed-off-by: Samuel Just <sam.just@inktank.com>
PG::_select_auth_object: prefer a peer which did not hit a read error

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