RFR: Set ShenandoahMinFreeThreshold default to 10%

[Aleksey Shipilev]

Our adopters report again that the default adaptive threshold of 3% is uncomfortably low to avoid
problems under allocation spikes. We have recommended at least two adopters to set it to 10%, and it
seems work for them fine.

Let's make that the real default:

diff -r 20f6d65431ec src/hotspot/share/gc/shenandoah/shenandoah_globals.hpp
--- a/src/hotspot/share/gc/shenandoah/shenandoah_globals.hpp	Wed May 02 21:42:13 2018 +0200
+++ b/src/hotspot/share/gc/shenandoah/shenandoah_globals.hpp	Thu May 03 11:05:19 2018 +0200
@@ -167,7 +167,7 @@
                "Initial remaining free threshold for adaptive heuristics")  \
           range(0,100)                                                      \
                                                                             \
-  experimental(uintx, ShenandoahMinFreeThreshold, 3,                        \
+  experimental(uintx, ShenandoahMinFreeThreshold, 10,                       \
                "Minimum remaining free threshold for adaptive heuristics")  \
           range(0,100)                                                      \
                                                                             \

In my performance runs, it does not regress performance, unless we are dealing with very tight heaps.

Testing: hotspot_gc_shenandoah, benchmarks

Thanks,
-Aleksey