CoreContainer Closer thread can miss cores

See for example http://jenkins.thetaphi.de/job/Lucene-Solr-master-Solaris/836/

The core T4 is placed on the 'to-close' queue by the transient core cache, but it never gets picked up by the Closer thread for some reason - I'm guessing it's a race somewhere, but the SolrCores code is really difficult to follow so I'm having trouble tracking down exactly where this is happening.