ReplicatedPG::do_op: delay if snapdir is unreadable

Since all we really need on a snapdir is the context, we really only
need it to be !missing.  However, it might become !missing before it
becomes !unreadable.  That allows ops to end up in the
waiting_for_degraded queue before one in waiting_for_unreadable is
woken, which allows the ops to be reordered.  Rather than reintroduce an
extra waiting_for_missing queue, simply require !unreadable for snapdir
(which implies !misssing).

Fixes: #7777
Signed-off-by: Samuel Just <sam.just@inktank.com>