RFR: Mark heuristics diagnostic/experimental

[Aleksey Shipilev]

We now have (N+1)-th adopter who used "aggressive" as the synonym for "good". While in reality, that
is our diagnostic heuristics, and it should not be used in production mode. Users' need is sometimes
justified, and we need another "continuous" heuristics to trigger continuous GC that is amenable to
shortcutting and does not evacuate as aggressively.

It is time for us to hide diagnostic/experimental heuristics better:
  http://cr.openjdk.java.net/~shade/shenandoah/heuristics-lock/webrev.01/

Only adaptive/dynamic are product heuristics now. Aggressive/passive are diagnostic. All partial
heuristics are experimental.

Testing: hotspot_gc_shenandoah (fastdebug/release)

Thanks,
-Aleksey