[URGENT] [8u] Backout "MetadataOnStackMark::record() is not MT-safe"

[Aleksey Shipilev]

There is a hard requirement for 8u-aarch64 integration that any non-mainline patch in shared code
was either trivial, or be obviously Shenandoah-specific. This patch was the attempt to fix the bug, 
and it still does not work. Therefore we are better to backout it completely.

This blocks 8u integration.

8u webrev:
   https://cr.openjdk.java.net/~shade/shenandoah/8u-backout-metadata/webrev.01/

This is a clean backout of:
   https://hg.openjdk.java.net/shenandoah/jdk8/hotspot/rev/b261d2db2ea2

Testing: hotspot_gc_shenandoah

-- 
Thanks,
-Aleksey