Memory leak when used in webapps

[TheAustinSwamy]

The Shutdown hook is registered twice in SshEngine when using default instance and both are not removed if the maverick client library is used in a web application thereby causing memory leaks.

SshEngine getDefaultInstance() method calls the startup() method and this method registers a shutdown hook.

Lines 242-243 (SshEngine.java)
if (Runtime.getRuntime() != null)
Runtime.getRuntime().addShutdownHook(shutdownHook);

SshEngine.getDefaultInstance() ALSO registers a shutdown hook. Lines 1203-1207.

During shutdown, only the first one is removed from the ApplicationHooks and thereby causing leaks.

The registration in Lines 1203-1207 of SshEngine.java is not necessary