RFR [10] 2018-03-12: Bulk backports to sh/jdk10

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/backports/jdk10-20180312/webrev.01/

This backports last week's changes to sh/jdk10.

46caecb58216: [backport] Use current time instead of DBL_MAX for immediate uncommit
eac60de937c2: [backport] Excessive assert in ShHeap::mark_next
b3446fc50093: [backport] Eliminate write-barrier assembly stub (part 1)
58279d273cc4: [backport] Degenerated GC: rename enum, report degen reasons in stats
0e32109905c8: [backport] Use os::naked_short_sleep instead of naked Thread events for sleeping
6d92c949ae62: [backport] No need for fence in control loop: flags are now ShSharedVariables
35f512e2f57f: [backport] Implement protocol for safe OOM during evacuation handling
125606c17be3: [backport] Minor cleanups
51fc1ad2dbff: [backport] Make control loop more responsive under allocation pressure
232c31483a6b: [backport] Split live data management for allocations and GCs

Testing: hotspot_gc_shenandoah

Thanks,
-Aleksey