improve performance of TextFileCertificateLoginModule when many entries are in the "textfiledn.user " file

With a large number of entries ( 200,000 ) in the "org.apache.activemq.jaas.textfiledn.user" file the performance seemed to degrade.To demonstrate the performance difference;| ||1) for 100 entries and calling initialize,login(),commit 10 times - Time taken is 73 miliseconds|

Test is attached.Suggested improvements:avoid loading the org.apache.activemq.jaas.textfiledn.user file each time - in PropertiesLoginModule.java, the file is only read when it changes, using the file modification time.avoid iterating through the Properties object, using a Map instead to retrieve the userName