Include Garbage Collection info in Diagnostics dump

When obtaining a Diagnostics Dump via `bin/nifi.sh diagnostics <filename>` the output should include information about garbage collection:

• Number of each type of garbage collection
• Average amount of time spent for each type of GC
• Longest GC pause encountered
• Info about each of the last N number of GC events, to include time/date, duration, reason for GC, type of GC, etc.