RFR 8038212: Method::is_valid_method() check has performance regression impact for stackwalking

[Coleen Phillimore]

Summary: Only prune metaspace virtual spaces at safepoint so walking 
them is safe outside a safepoint.

Walking class loader data graph for contains is really slow for 
applications like nashorn that has a lot of class loader data.

Tested with nsk.quick.testlist, jtreg tests, and jck tests.  Also ran

open webrev at http://cr.openjdk.java.net/~coleenp/8038212/
bug link https://bugs.openjdk.java.net/browse/JDK-8038212

Marcus L. tested it with nashorn tests.

Thanks,
Coleen