Checkpoints.getOldestRevisionToKeep shouldn't failed if called read-only

  java.lang.UnsupportedOperationException: Method - findAndUpdate. Params: [settings, key: checkpoint update {data.r17cea1494d3-0-1=REMOVE_MAP_ENTRY null}]
  	at org.apache.jackrabbit.oak.plugins.document.util.ReadOnlyDocumentStoreWrapperFactory$1.invoke(ReadOnlyDocumentStoreWrapperFactory.java:38)
  	at com.sun.proxy.$Proxy0.findAndUpdate(Unknown Source)
  	at org.apache.jackrabbit.oak.plugins.document.Checkpoints.getOldestRevisionToKeep(Checkpoints.java:149)
  	at org.apache.jackrabbit.oak.plugins.document.VersionGCRecommendations.<init>(VersionGCRecommendations.java:181)