RFR: [9] 2018-02-27: Bulk backports to sh/jdk9

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/backports/jdk9-20180227/webrev.01/

This backports the all-important bug fixes to sh/jdk9:

d3dcc4565dcb: [backport] String dedup thread's poll() can interfere with scanning at safepoint
2a2fe8eff7cc: [backport] ReferenceProcessor is_alive setup is racy
aea30526d0a4: [backport] Speed up asserts and verification, improve fastdebug builds performance

Testing: hotspot_gc_shenandoah {fastdebug|release}

Thanks,
-Aleksey