idmanager fails with TransactionCommitFailedException Caused by: java.lang.IllegalStateException: Store tree ... and candidate base ... differ.

While working on the new style end2end component test for netvirt elan (ElanServiceTest) in https://git.opendaylight.org/gerrit/#/c/44000/, I've hit an "TransactionCommitFailedException Caused by: java.lang.IllegalStateException: Store tree ... and candidate base ... differ" in genius idmanager; full log with stack trace provided in attachment.

This looks similar to CONTROLLER-1578 - but that one is in elanmanager itself, whereas this one is in idmanager; thus filing this as a separate bug.

GENIUS-49 does not fix this - I've locally verified it.

This is easily reproducible; grab patch set 12 of c/44000 (https://git.opendaylight.org/gerrit/#/c/44000/12), uncomment the two lines re. RunUntilFailureRule at the top of ElanServiceTest, and let it run - and this will happen.