Tuning graphloader JVM options

The DSE Graph Loader is written in Java and has some configurable JVM tuning in the graphloader script.

The default maximum heap size is 10G, generally a good heap size for appropriately sized machine used with graphloader. Two environment variables, MAX_HEAP_SIZE and HEAP_NEWSIZE were added in DSE 5.0.5 and later. graphloader now calculates the values of these two environment variables in the same manner as the DSE database. If a particular value is desired for either variable, the value can be set directly in the graphloader script.