RFR: Disable suspendible workers (again)

[Roman Kennke]

I recently enabled suspendible workers because I suspected running
workers would conflict with deoptimization. This has proved wrong, and I
would like to disable it (by default) again. We shall leave it in
though, as kind of red button, in case anybody ever encounters a problem
with GC workers during non-GC-safepoints. In order to avoid bitrotting
this code, I added a few tests to run with +STS

Testing: tier3_gc_shenandoah

http://cr.openjdk.java.net/~rkennke/disable-sts/webrev.00/

OK?

Roman