osd: provide better version bounds for cls_current_version and ENOENT replies

Following the changes to when we set or increase the user_version, we
want to continue to return the best lower bound we can on the version
of any newly-created object. For ENOENT replies that means returning
info.last_user_version instead of the (potentially-zero) ctx->user_at_version.

Similarly, for cls_current_version we want to return the last version on
the PG rather than the last update to the object in order to provide
sensible version ordering across object deletes and creates.

Update the versions doc so it continues to be precise.

Signed-off-by: Greg Farnum <greg@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>