10 RFR: 8169517: WhiteBox should provide concurrent GC phase control

[Kim Barrett]

Please review this addition to the WhiteBox testing infrastructure,
providing an API for putting a concurrent collector into a particular
concurrent phase.

This is for writing tests where some action of the test needs
to be performed while the collector is in a specific state. For
example, to test a fix for JDK-8166188 we will need to dereference a
jweak while G1 is doing concurrent marking. It is difficult to
reliably arrange for that to happen without direct controls like those
being provided here.

Only G1 support for the new feature is provided by this change. CMS
support is TBD, and will be the subject of a new RFE.

CR:
https://bugs.openjdk.java.net/browse/JDK-8169517

Webrev:
http://cr.openjdk.java.net/~kbarrett/8169517/hs.00/
http://cr.openjdk.java.net/~kbarrett/8169517/hotspot.00/

Testing:
Change includes jtreg tests for the new feature.
Local and jprt-based jtreg testing.