RFR JDK-8187653: Lock in CoderResult.Cache becomes performance bottleneck

[Xueming Shen]

To follow Claes's suggestion to make the CoderResult.Cache.cache field 
final and allocate early.

issue: https://bugs.openjdk.java.net/browse/JDK-8198966
webrev: http://cr.openjdk.java.net/~sherman/8198966/webrev/

Thanks,
-Sherman