Revert GC thread changes

This is not quite a straight revery, some manual edits were necessary.

The original CL didn't undergo sufficient design review or testing.  Revert until the regressions can be sorted out.

Bug 8585185


This reverts commit 6dacf835

This reverts commit f8c033db

Change-Id: Ie7215bdf881332e822603547e92f810f595077fc