RFR (S): 8231189: Rename worker_i parameters to worker_id

[Thomas Schatzl]

Hi all,

   can I have reviews for this change that renames all remaining 
"worker_i"s to "worker_id"s?

In the jdk9 timeframe we did quite a few renamings of similarly used 
variables to "worker_id", but either some "worker_i" crept in or were 
forgotten.

This rectifies this.

CR:
https://bugs.openjdk.java.net/browse/JDK-8231189
Webrev:
http://cr.openjdk.java.net/~tschatzl/8231189/webrev/
Testing:
Local compilation

(Patch based on JDK-8159984 also out for review)

Thanks,
   Thomas