RFR: Refactor gc+init logging

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/gc-init-msg/webrev.01/

Let's make it more concise:

[0.002s][info][gc,init] Regions: 2560 x 4096K
[0.002s][info][gc,init] Humongous object threshold: 4096K
[0.002s][info][gc,init] Max TLAB size: 4096K
[0.002s][info][gc,init] GC threads: 13 parallel, 13 concurrent
[0.002s][info][gc,init] Reference processing: parallel
[0.002s][info][gc,init] Shenandoah heuristics: adaptive
[0.003s][info][gc,init] Safepointing mechanism: global-page poll

Testing: tier3_gc_shenandoah

Thanks,
-Aleksey