RFR: Avoid superfluous proactive GCs

[Per Liden]

When running applications with a very small live-set and very low 
allocation rate there's a tendency to do proactive GCs a little too 
often. An idling jshell instance is an example of an application where 
this can be seen, where we're doing GCs like every 5 second or so, which 
is just annoying. With this patch we only consider doing proactive GCs 
if the app has eaten some memory (at least 10%) or enough time has 
passed (5 min).

http://cr.openjdk.java.net/~pliden/zgc/avoid_superfluous_proactive_gcs/webrev.0/

cheers,
Per