Long and redundant classpaths for restarted Stages

[laserval]

Note: Still being investigated

If a stage is restarted many times, the classpath used for its startup command will grow with each restart.

This could be due to the classpathString property of the StageRunner never being reset, and all calls to getClasspath() appending the jars to the current classpath.

https://github.com/Findwise/Hydra/blob/master/core/src/main/java/com/findwise/hydra/StageRunner.java#L239

[laserval]

It appears this was fixed with 0a38bd6