Shenandoah: Simplify and enhance reporting of requested GCs

Shenandoah distinguishes between 'implicit' and 'explicit' GC requests. The distinction is used to decide whether or not a request is serviced concurrently or at a full GC. The data collected is also included in the end-of-process report. This change simplifies handling these requests and adds a tally of the underlying GC causes to the end-of-process report.