Option to use symlinks for source and classes

[mgor]

The use case is a big application (more than 1 Gb, less than 2 gb), which collects metrics via jacoco-agent every night, to get statitics of functional test coverage.

When the source and classes is copied for each job, the disk space usage get quite big over time.

When it's only the metrics changing, and not the classes and source, it makes more sense to use symbolic links for these in the job directory instead of a copy.

[mgor]

There will be some merge conflicts with PR #95.

If PR #95 is accepted, I'll push a new version of this one.

[centic9]

https://issues.jenkins-ci.org/browse/JENKINS-50108