RFR(L) 8237354: Add option to jcmd to write a gzipped heap dump

[Schmelter, Ralf]

Hi Yasumasa,

I think I've tried too much by using the -gz flag as a boolean plus a int value. I've decided to use two options instead: -gz as a boolean option to turn compression on and -gz-level to specify the compression level. E.g. 
GC.heap_dump -gz -gz-level=3 test.hprof.gz

Best regards,
Ralf