Make cache directory size configurable

[gochujang-c]

At the moment, it is hardcoded on 128G, see:

dj-pcfg/src/main/java/nl/nfi/djpcfg/guess/cache/distributed/CheckpointFileCache.java

Line 36 in c1cf889

public static long MAX_CACHE_SIZE = 128L * 1024 * 1024 * 1024;

It would be good to make this a runtime configuration