RFR: Make Shenandoah SATB mutex ranks consistent with G1

[Roman Kennke]

Apparently, upstream changed the ranks of the SATB mutexes from special 
to access. We missed to do the same. Fix:

http://cr.openjdk.java.net/~rkennke/satbmutex/webrev.00/ 
<http://cr.openjdk.java.net/%7Erkennke/satbmutex/webrev.00/>

To be honest, the real fix would be to not declare those mutexes 
globally, but put it under satbQueue.cpp where they are actually used. 
But this would have to be done upstream.

Ok to push?

Roman