RFR (S) 8243238: Shenandoah: explicit GC request should wait for a complete GC cycle

[Roman Kennke]

> Bug:
>   https://bugs.openjdk.java.net/browse/JDK-8243238
> 
> See the discussion in the bug itself.
> 
> Webrev:
>   https://cr.openjdk.java.net/~shade/8243238/webrev.01/
> 
> Testing: hotspot_gc_shenandoah, DirectBufferAllocTest

Why do you call reset_gc_id() in the ctor instead of initializing it to
0 in the regular initializer?

Don't we have a global GC ID counter already, somewhere?

Other than that, the patch looks good, and fixes the problem for me (I
could reproduce it almost 100% before).

Roman