Fix workunit parent assignment issue.

Previously, even when passing in an explicit parent,
the workunit would get incorrectly set as the current workunit
for the calling thread.

Also make sure symlinking of ivy resolve jars happens
after we attempt to cache a classpath referring to those
symlinks.

Auditors: pl
Fix spurious missing jardep messages.

Also map jar symlinks when interpreting the ivy report.

Auditors: pl
Ensure jar symlinking even when reading from artifact cache.

Also gets rid of a couple of superfluous members of IvyUtils.

Auditors: pl
Merge branch 'master' into full_build_refactor

Conflicts:
	src/python/twitter/pants/tasks/jvm_dependency_analyzer.py