Today in 6.x it is possible to add an index tombstone to the graveyard without
deleting the corresponding index metadata, because the deletion is slightly
deferred. If you shut down the node and upgrade to 7.x when in this state then
the node will fail to apply any cluster states, reporting

    java.lang.IllegalStateException: Cannot delete index [...], it is still part of the cluster state.

This commit addresses this situation by skipping over any index metadata with a
corresponding tombstone, allowing this metadata to be cleaned up by the 7.x
node.