Set initial collection time in gc monitor
Set initial collection time in gc monitor

The java GarbageCollectorMXBean.getCollectionTime returns the cumulative
amount of time the collector has run during the jvm session. The GC
monitor is tracking how much time has been spent in garbage collection
during each task evaluation run. In order for this calculation to work
correctly, it is necessary to set the initial elapsed time to the bean's
current collection time when we create the gc monitor. Without doing
this, we can get completely incorrect results that are reporting based
on the total gc time for the entire process, not just in the last 10
seconds.

Should fix https://github.com/sbt/sbt/issues/5818

mdedetrich/sbt,mdedetrich/sbt,sbt/sbt,xuwei-k/xsbt,mdedetrich/sbt,xuwei-k/xsbt,xuwei-k/xsbt,sbt/sbt,sbt/sbt,mdedetrich/sbt,sbt/sbt,sbt/sbt,mdedetrich/sbt,xuwei-k/xsbt,xuwei-k/xsbt,sbt/sbt,mdedetrich/sbt,sbt/sbt,mdedetrich/sbt