Repository/branch for development of generational ZGC

[Stefan Karlsson]

FYI, we have just created a development branch with an initial set of 
changes to start building generations into ZGC:
https://github.com/openjdk/zgc/tree/zgc_generational

We've been experimenting with this feature for while; testing different 
barriers, remember set handling, GC cycle synchronization, etc. The code 
passes most of our tier1-tier7 testing (linux-x64 only), with exception 
for some serviceability features. Note, that this is still 
work-in-progress, and we are going to make a a lot of changes. Just as 
an example, there are currently no GC heuristics in place to trigger 
minor and major cycles; they are simply triggered based on timers.

StefanK