RFR 7004967: SecureRandom should be more explicit about threading

[Weijun Wang]

Please review the enhancement at

   http://cr.openjdk.java.net/~weijun/7004967/webrev.00/

Basically, we want SecureRandom to be more efficient by removing all 
synchronized keywords from its public methods and let an implementation 
to take care of thread-safety (We already did some in JDK-8098581). On 
the other hand, we need to make sure that existing implementations that 
have not synchronized correctly to behave just as good as before.

Therefore a new Service Attribute "ThreadSafe" is introduced. If you 
think your implementation is already thread-safe, set it to "true" and 
SecureRandom will be happy. Otherwise, don't set it and SecureRandom 
will continuously call your SecureRandomSpi engine methods in 
synchronized blocks.

Thanks
Max