IMPT: Switching development to shenandoah-jdk10

[Aleksey Shipilev]

Starting today, we are switching the regular development to:
  http://hg.openjdk.java.net/shenandoah/jdk10

I have pulled all recent work there, and merged it with the current state of JDK
10 upstream. Please rebase your pending work, if any, to shenandoah-jdk10, and
only push there.

shenandoah-jdk9 now joins shenandoah-jdk8u in being the backports-only
repository. We would do the stabilization work for JDK 9 release there, while
continuing to develop potentially breaking changes in shenandoah-jdk10.

Thanks,
-Aleksey