RFR: Factor out keep-alive barrier

[Roman Kennke]

Turns out that for conc-partial we also need different handling for
objects that we need to keep alive for some reason. This mostly affects
Reference.get() calls, but also some stuff in the runtime. This patch
creates an abstraction for this type of barrier, and lets the
implementations call the usual SATB pre-barrier. (Conc-partial will do
something else there).

Normally I'd consider this something to be upstreamed. However, Erik and
I are currently on a different, more complex and more powerful solution,
which would obsolete this soon. But I need something now for conc-partial.

Tests: hotspot_gc_shenandoah

http://cr.openjdk.java.net/~rkennke/keepalivebarrier/webrev.00/

Ok?