Fix MemKeyValueStore further concurrency issue

java.util.ConcurrentModificationException
	at java.util.TreeMap$PrivateEntryIterator.nextEntry(TreeMap.java:1211)
	at java.util.TreeMap$KeyIterator.next(TreeMap.java:1265)
	at net.jodah.expiringmap.ExpiringMap$EntryTreeHashMap$AbstractHashIterator.getNext(ExpiringMap.java:473)
	at net.jodah.expiringmap.ExpiringMap$EntryTreeHashMap$KeyIterator.next(ExpiringMap.java:491)
	at java.util.Iterator.forEachRemaining(Iterator.java:116)
	at java.util.Spliterators$IteratorSpliterator.forEachRemaining(Spliterators.java:1801)
	at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:481)
	at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:471)
	at java.util.stream.ForEachOps$ForEachOp.evaluateSequential(ForEachOps.java:151)
	at java.util.stream.ForEachOps$ForEachOp$OfRef.evaluateSequential(ForEachOps.java:174)
	at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234)
	at java.util.stream.ReferencePipeline.forEach(ReferencePipeline.java:418)
	at org.nuxeo.runtime.kv.MemKeyValueStore.keyStream(MemKeyValueStore.java:67)
	at org.nuxeo.ecm.core.bulk.BulkServiceImpl.await(BulkServiceImpl.java:248)
	at org.nuxeo.ecm.core.bulk.CoreBulkFeature.lambda$initialize$0(CoreBulkFeature.java:45)
	at org.nuxeo.runtime.test.runner.TransactionalFeature.await(TransactionalFeature.java:121)
	at org.nuxeo.runtime.test.runner.TransactionalFeature.nextTransaction(TransactionalFeature.java:101)
	at org.nuxeo.runtime.test.runner.TransactionalFeature.nextTransaction(TransactionalFeature.java:77)
	at org.nuxeo.ecm.platform.importer.tests.TestImporterWithFileManager.testImporterContribution(TestImporterWithFileManager.java:83)