ZGC: Adjust warmup criteria

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" GCs until the GC is warm), but instead only consider the GC to be warm once we've done three "Warmup" GCs.