RFR [8], 2018-09-03, Bulk backports to sh/jdk8

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/backports/jdk8u-20180903/webrev.01/

This backports recent work to sh/jdk8:

 [backport] GC trace messages have to be immortal
 [backport] Wiring GC events to JFR + Restore heap occupancy in GC logs after JFR changes
 [backport] Remove obsolete/unused logging usages
 [backport] Replace custom asserts with shenandoah_assert_*
 [backport] Wiring heap and metaspace info to JFR
 [backport] Out-of-cycle Degenerated GC should process references and unload classes
 [backport] Off-by-one error in degen progress calculation
 [backport] Only Java and GC worker threads should get GCLABs
 [backport] Move ParallelCodeIterator to ShenandoahCodeRoots
 [backport] Evac reserve: make sure GC has untouchable space to move the objects into
 [backport] Refactor FreeSet logging: support evac-reserve, denser printouts
 [backport] Enable ShenandoahEvacReserveOverflow by default

Testing: hotspot_tier3_gc_shenandoah {fastdebug|release}

Thanks,
-Aleksey