OOM Errors in 8GB JVM builds, -XX:SoftMaxHeapSize

[Fabian Huch]

We are recently seeing OOM errors for builds with 8GB heap memory, e.g.: 
https://build.proof.cit.tum.de/build?name=user%2F2212

Since we are using the Z garbage collector, I've now changed the build 
manager to use its soft heap max size (8g) with a hard limit of 32g.

This should mean that we won't see any OOM errors unless things really 
go wrong. However, it is a bit concerning that we went from <4g to >8g 
in the last few months.

If the soft heap limit works well, we could also do this by default in 
the distribution.


Fabian