Review Request: JDK-8235521: Replacement API for Unsafe::ensureClassInitialized

[Mandy Chung]

This proposes a new `Lookup::ensureClassInitialized` API as a replacement
for `sun.misc.Unsafe::ensureClassInitialized`.The Lookup object must have
the access to the given class being initialized.

CSR: https://bugs.openjdk.java.net/browse/JDK-8245871

webrev:
http://cr.openjdk.java.net/~mchung/jdk15/webrevs/8235521/webrev.01/

This patch converts the JDK use of 
`jdk.internal.misc.Unsafe::ensureClassInitialized` to
call this new API where appropriate. `Unsafe::ensureClassInitialized` 
remains for java.lang.invoke.* internal implementation use.

A framework can use `Lookup::privateLookupIn` to access a class in 
another module if the module authorizes it by opening the packages for 
it to access. Or a user can produce a less privileged lookup by 
Lookup::dropLookupMode and pass it to the framework.

`Class::ensureInitialized` was considered as an alternative. The 
downside for this option is that it would be caller sensitive in order 
to ensure the caller has the proper access to this class.  And it may 
not work for frameworks which have no access to user classes. It would 
have to fallback toClass::forName` limited solution.

Thanks
Mandy