RFR[S] Refactor HeapShared::archive_reachable_objects_from_static_field

[Ioi Lam]

https://bugs.openjdk.java.net/browse/JDK-8210295
http://cr.openjdk.java.net/~iklam/jdk12/8210295-refactor-heapshared-archive-static-field.v01/

Background
----------

Currently, the only way to archive an object subgraph is to call
HeapShared::archive_reachable_objects_from_static_field, which
requires the root to be in a static field of a Java class.

To support JDK-8198698 (Archive Lambda classes in CDS), we need to
archive subgraphs from other sources, such as objects stored in
ConstantPool::resolved_references().


Fix
---

This is intended to be a clean-up only, with no change in functionality.

I moved the common code into a new method 
HeapShared::archive_reachable_objects_from()
In doing so, I was able to consolidate some code that was duplicated between
archive_reachable_objects_from_static_field and WalkOopAndArchiveClosure.


Also:

+ removed some obsolete comments (e.g., "only ArchivedModuleGraph is 
supported")
+ moved excessive tracing code from Debug to Trace
+ improve the alignment of some logs to make them more readable

Thanks
- Ioi