RFR: 8232001: ZGC: Ignore metaspace GC threshold until GC is warm

[Per Liden]

As reported here:

https://mail.openjdk.java.net/pipermail/zgc-dev/2019-September/000736.html

The ZDirector heuristics can get of to a bad start if the statistics is 
contaminated by early "Metaspace GC Threshold" GC requests. To avoid 
this, we could simply ignore such requests until the GC is warm, at the 
potential cost of expanding metaspace a bit more during startup.

Bug: https://bugs.openjdk.java.net/browse/JDK-8232001
Webrev: http://cr.openjdk.java.net/~pliden/8232001/webrev.0

/Per