RFR: 8234312: ZGC: Adjust warmup criteria

[Per Liden]

JDK-8232001 introduced logic which ignores "Metastace GC Threshold" 
collections until the GC has warmed up. While this works in principle, 
it also causes some intermittent metaspace test failures. These tests 
use a small MaxMetaspaceSize, and aggressively loads and unloads 
classes. It's not completely obvious why these tests started to fail 
after JDK-8232001. However, we will be collecting metaspace a little bit 
later now, which could mean we have more fragmentation and are not able 
to free as much memory when the GC eventually happens.

This patch reverts to the old behavior (we no longer ignore "Metspace GC 
Threshold" collections until the GC is warm), but instead we only 
consider the GC to be warm once we've done three "Warmup" GCs.

Bug: https://bugs.openjdk.java.net/browse/JDK-8234312
Webrev: http://cr.openjdk.java.net/~pliden/8234312/webrev.0

Testing: Tier1-7 on ZGC. Multiple manual runs of 
vmTestbase/gc/gctests/LoadUnloadGC (the test that intermittently fails 
in our CI).

/Per