RFR: Take boost mode into account when calculating duration of gc

[Per Liden]

When using boost mode (i.e. when we're using all available worker 
threads instead of ConcGCThreads) we typically complete a GC cycle much 
faster than normal (as intended). However, this causes the "duration of 
gc" calculations to be artificially deflated, can later lead to a 
situation where we risk starting a GC cycle too late. This patch 
corrects the "duration of gc" calculation by taking the "boost factor" 
into account.

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

/Per