Improve Java 9 support

[henri-tremblay]

After playing a bit with Java 9, here is what I've found:

• Java 9 can't load the agent. So UnsafeSizeOf is used
• Then it works well excecpt: We get Illegal access warnings from ObjectGraphWalker

Also, Java 9 is using less memory (good!) so we need to adapt our tests accordingly.

[henri-tremblay]

Some really interesting answers from the jdk9 mailing list:

• Self-attach is not allowed anymore
• To circumvent it, you can for instance look at what Byte Buddy does, or simply depend on byte-buddy-agent to get an instance of Instrumentation by doing inst = ByteBuddyAgent.install();
  For the code, see https://github.com/raphw/byte-buddy/tree/master/byte-buddy-agent
• Add -Djdk.attach.allowAttachSelf=true
• Another way is to use ProcessBuilder to create another process and have the new process attach. This follows all of the current rules. Several projects are using this approach.

[henri-tremblay]

If the agent get loaded correctly, it can also be used to open modules (including the JDK) which should allow ObjectGraphWalker to work.