OPENDJ-3046 Cleanup server offline tools environment

When the various tools run in offline mode they initialise some parts of the directory server in order to perform their tasks.
Usually the tool will exit when it has finished processing and the state of this environment is not a problem.
However, if the tool is used within another process then some initialised components may be left over (e.g ads-trustore or admin data backends).
Extract a method from the DirectoryServer#shutdown() to shutdwon backends only and use it straight after TaskTool#processLocal.