Fix BAF action's query in charge of retrieving expired documents

TestRetentionManager.testRetainUntilDateSaved fails randomly:

java.lang.AssertionError
	at org.junit.Assert.assertNotNull(Assert.java:722)
	at org.nuxeo.retention.test.TestRetentionManager.testRetainUntilDateSaved(TestRetentionManager.java:218)

On the line assertNotNull(saved).

It's likely that on slow machines something takes too long.

EDIT the query used to retrieve the documents of which the retention date expired rounds the now date down to the lower second i.e. no millisecond precision. As a direct result, the unit tests playing with retention periods of few hundred of milliseconds, we have edge cases where some expired documents are not caught by the BAF action in charge of notifying the retention expired.