Fix: attrd: avoid race condition at writer election

f5263c94 was not a complete fix. The issue may also occur if a remaining node
(not the original DC or writer) wins the attribute writer election after the
original DC's controller has exited but before its attribute manger has exited.

The long-term solution will be to have the attribute manager (instead of the
controller) be in control of erasing transient attributes from the CIB when a
node leaves. This short-term workaround simply has new attribute writers skip
shutdown attributes when writing out all attributes.

Fixes T138