Confluence should used fixed heap sizes for shutdown scripts

Heap sizes for Confluence are set in the system-appropriate SETENV file.
While it makes sense for Confluence to use the specified heap size when starting a new Confluence instance, the shutdown script also uses the minimum heap size.

In case where the minimum heap size is quite large (>50% of physical memory), the shutdown script will be unable to allocate memory when it tries to start. The solution is to have the shutdown script use a fixed minimum necessary amount of heap.
This would prevent users from having to reduce the heap size in setenv temporarily just to run the shutdown script, then reinstate the original heap size when starting Confluence up again.