Reduced set of classes that are initialized at build time by Native Image

[Vojin Jovanovic]

Hi all,

We plan to reduce the set of classes that are initialized at build time by default in GraalVM Native Image. We plan to do this because for many JDK packages (e.g., `java.awt’) the cost of maintaining initialization at build time outweighs the performance benefits. The issue that covers this change is 
 
 https://github.com/oracle/graal/issues/2650

and the corresponding PR is 

 https://github.com/oracle/graal/pull/2656

The plan is to merge this feature at the end of the week (before the cut-off for the 20.2 release). I will try to shrink the list of initialized packages even further (https://github.com/oracle/graal/pull/2656/files#diff-fe1c43eaa0fc9417273054562118a946). However, future changes of this list should be small compared to what is already done.

This might be a breaking change for any project that uses Native Image and initializes classes at build time: JDK classes that got accidentally initialized will report an error, and JDK objects that end up in the image heap will report an error. 

The fix should be straightforward: either don’t initialize your project classes at build time, or simply add the initialized (or snapshotted) classes to the list of initializations with `—initialize-at-build-time`. 

Please comment on the PR if you have ideas, concerns, or unexpected issues with your project. 

Thanks,
- Vojin