[15] 8242400: Shenandoah: Restore logging to pre-jdk8241984 format

[Zhengyu Gu]

JDK-8241984 changed logging format, which is not desirable.
This patch intents to restore old format:

For example:

...

0.261s][info][gc,start     ] GC(0) Concurrent roots processing
[0.261s][info][gc,task      ] GC(0) Using 2 of 4 workers for concurrent 
root processing
[0.262s][info][gc           ] GC(0) Concurrent roots processing 0.478ms
[0.262s][info][gc,start     ] GC(0) Concurrent cleanup
[0.268s][info][gc           ] GC(0) Concurrent cleanup 68M->12M(256M) 
5.745ms
[0.268s][info][gc,ergo      ] GC(0) Free: 230M, Max: 256K regular, 174M 
humongous, Frag: 25% external, 0% internal; Reserve: 13312K, Max: 256K

...


Bug: https://bugs.openjdk.java.net/browse/JDK-8242400
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8242400/webrev.00/index.html

Test:
   hotspot_gc_shenandoah
   Eyeball outputs
   Eyeball JMC events

Thanks,

-Zhengyu