RFR 8064890: SecureClassLoader should use a ConcurrentHashMap

[Weijun Wang]

This fix is also a part for JEP 232 (Improve Secure Application 
Performance) [1].

webrev: http://cr.openjdk.java.net/~weijun/8064890/webrev.00/
bug: https://bugs.openjdk.java.net/browse/JDK-8064890
The fix uses a ConcurrentHashMap to avoid synchronization.

Note:

1. ConcurrentHashMap does not accept null keys.

2. Lambda cannot be used at such an early stage.

Noreg-cleanup.

Thanks
Max


[1] http://openjdk.java.net/jeps/232