Support maximum heap size system property

The Java maximum heap size may be specified using a system property.

There is no validation of the input maximum heap size except to ensure that
it does not contain whitespace. The input maximum heap size, if valid, is
appended to -Xmx= and added to the java command line returned from
release.

Add maximum heap size documentation to the README.

Name maximum heap size methods etc. heap_size_maximum given that there
is also a minimum heap size Java command line parameter. This will
make for a more logical list if methods etc. are sorted.

[#49694395]
Merge 49694395-heap-size to master

[Completes #49694395]