Leak: reported memory leak when etcd read available only

A user reported a memory leak in fleet and locksmith and I can reproduce. Since both of these components seem to be leaking and they both use etcd it seems like there must be an unknown leak in the read available only code paths.

To reproduce:

1) Setup a cluster of three vagrant machines
2) Kill etcd on two of them
3) Watch for leaking, it is easy to reproduce with locksmithctl send-need-reboot on a machine with etcd read available.

/cc @bcwaldon @jonboulle