RFR (S): JDK-8031116 : Fix thread-id types in G1 remembered set implementations

[Sangheon Kim]

Hi all,

   can I have reviews for the following small change that changes the
uses of the "int" type for thread ids into uints like everywhere else?

Thomas agreed to sponsor the change.

CR:
https://bugs.openjdk.java.net/browse/JDK-8036116

Webrev:
http://cr.openjdk.java.net/~tschatzl/8036116-kim/webrev.00/ 
<http://cr.openjdk.java.net/%7Etschatzl/8036116-kim/webrev.00/>

Testing: jprt

Thanks,
Sangheon