[toolchain] create "maxmem" setting for all plugins to refer to

Create a centralized setting for the max memory setting that all plugins launching a new JVM to respect/use. Then, update the plugins to use it.

With the time spent repeatedly running, researching, and configuring some plugins to not have "Out of Memory" errors, it now seems silly not to add this kind of feature in the revolutionary vs evolutionary Maven 2!

I would prefer a setting at the core Maven level, e.g. maven.plugins.maxheap=1024m, that the plugins "magically" used, vs having to repeatedly specify the same one to each plugin that needs it.