Investigate invalid operation

I recommend viewing this pull request with ?w=1 attached to the url.

I think the exception in http://cislave11.incubaid.com/jenkins/jobs/jenkins-arakoon-1.6-system-tests-slow-RIGHT-83/home/jenkinslxc/qbase3/var/tmp/arakoon_system_tests/test_sso_deployment/sturdy_0/log/sturdy_0.debug.1370002695.000000 came through on_consensus (https://github.com/Incubaid/arakoon/blob/1.6/src/node/store.ml#L850 ) untill https://github.com/Incubaid/arakoon/blob/1.6/src/node/store.ml#L807 where it failed on the _get_j call.
That path doesn't check store.closed (only store.quiesced), so maybe we want to check that there too?
Or is it good enough to die with a message saying the store was closed (added exception wrapping for that)?