It would be better to find out why the nextBuildNumber is at times empty.
In the mean time this will find the appropriate number and write it to the file.

getBuildingUpstream() to public so they can be accessed from plugins,
small refactoring (reuse getTransitiveDownstreamProjects() and
getTransitiveUpstreamProjects() respectively)

This lets us get rid of Xalan safely (as well as other often problematic
dependencies like nekohtml + Xerces combo), and reduce the dependency
footprint.

don't wake up so often when waiting for Jenkins to finish - it make take away to much time from the threads that are actually doing something

[JENKINS-11416] Added comment describing the motivation of the change. The real fix is in: ccdbd834

Adding the ant-launcher jar to the maven plugin.  On my environment without it gmaven is unable to locate the org.apache.tools.ant.launch.AntMain class

[JENKINS-11458] Channel.current() is only available from the thread that's executing the remote call, and that's why we saw this original error, when Maven uses a thread pool to call into this method from another worker thread.

Fix minor typo in javadocs.