RFR: 8241700: Shenandoah: Fold ShenandoahKeepAliveBarrier flag into ShenandoahSATBBarrier

[Roman Kennke]

Keep-alive of weak references and similar is strongly bound with SATB.
There seems no reason to have two flags. Let's fold them and eliminate
one flag.

Bug:
https://bugs.openjdk.java.net/browse/JDK-8241700
Webrev:
http://cr.openjdk.java.net/~rkennke/JDK-8241700/webrev.00/

Testing: hotspot_gc_shenandoah

Good?

Roman