RFR: 8234440: ZGC: Print relocation information on info level

[Per Liden]

When using -Xlog:gc*, I now and then find that I miss basic relocation 
information, since it's currently printed at the debug level on the 
relocation set selector. I think we should leave the current logging as 
is, since that's still useful when debugging the relocation set selector 
itself. However, I think we should propagate some of the high level 
information and print it on the info level.

Here's an example of what the output looks like with this patch:

[...]
[68.926s][info][gc,reloc    ] GC(6) Small Pages: 529 / 1058M(93%), 
Empty: 350M(31%), Compacting: 450M(40%)->20M(2%)
[68.926s][info][gc,reloc    ] GC(6) Medium Pages: 2 / 64M(6%), Empty: 
0M(0%), Compacting: 64M(6%)->32M(3%)
[68.926s][info][gc,reloc    ] GC(6) Large Pages: 2 / 12M(1%), Empty: 
6M(1%), Compacting: 0M(0%)->0M(0%)
[68.926s][info][gc,reloc    ] GC(6) Relocation: Successful
[...]

Bug: https://bugs.openjdk.java.net/browse/JDK-8234440
Webrev: http://cr.openjdk.java.net/~pliden/8234440/webrev.0

/Per