Increase the default maximum heap and direct memory sizes

The default maximum heap size in Nexus Repo 3 is currently 1.2Gb.  The reason for this is historical. Back when we supported 32 bit Windows in Nexus Repo 2 that was the highest heap size that could reliably be allocated when memory was fragmented.

There is no good reason for the default heap to be that small these days, and It is becoming very clear that this is not an adequate heap allocation for Nexus Repo 3. We constantly see people who have problems after upgrading to a new version because they forget to adjust heap memory size from the default values.

I propose that we make the sizing profiles here 8, 16, and 32 (increasing from the current 4, 8, and 16):

https://help.sonatype.com/display/NXRM3/System+Requirements#SystemRequirements-GeneralMemoryGuidelines

And make the heap and direct memory defaults what is shown for the 8Gb model.