CMS: reduce perm heap bloat on account of 4957990

Note that the fix for 4957990 will fix the perm heap bloat whenever
a stop world collection is done.

This bug will address the same question in the context of
concurrent class unloading by CMS. This part of the work is
being split off into a separate CR to expedite the integration
of the fix for 4957990 for other collectors without holding them
hostage to the concurrent class unloading in CMS.

See 4957990 for a test case and other analysis.