RFR: 8035284: (xs) Remove redundant null initialization

[Mike Duigou]

Hello all;

This is a simple cleanup changeset that removes redundant initialization of fields to null from a number of collection classes. These field initializations may seem cheap but they do have a cost:

- For volatile fields there is a measurable cost on some benchmarks for these extra initializations.

- Larger byte code in <init> methods.

- For transient fields the initialization is misleading since it does not occur on deserialization.

https://bugs.openjdk.java.net/browse/JDK-8035284
http://cr.openjdk.java.net/~mduigou/JDK-8035284/0/webrev/

Redundant null initializations in other components/packages will be handled in separate issues.

Mike