Compile using JDK 11

As discussed on wildfly-dev mailing list, to enable the use of multi-release wildfly modules, we will:

• Require JDK 11 to compile (for all modules except testsuite modules)
  • Adjust compiler flags to use --release=8
    • This is equivalent to: -source N -target N -bootclasspath <bootclasspath-from-N>
• To ensure surefire tests are run against each supported JRE (wherever necessary), we will update the CI server maven settings to specify system properties for:
  • java8.home
  • java11.home
• Update JDK8 specific CI servers to build with JDK 11, but run integration tests using Java 8.

One complication is the wildfly-cli module, which contains a dependency on the jdk.jconsole module.  Building on JDK 11 using -release 8 means that we cannot just include the --add-modules=jdk.jconsole compiler argument, we actually need to add a jar containing a jconsole stub to the classpath of the compiler.