RFR: 8254598: StringDedupTable should use OopStorage

[Kim Barrett]

Please review this change to the String Deduplication facility.  It is being
changed to use OopStorage to hold weak references to relevant objects,
rather than bespoke data structures requiring dedicated processing phases by
supporting GCs.

(The Shenandoah update was contributed by Zhengyu Gu.)

This change significantly simplifies the interface between a given GC and
the String Deduplication facility, which should make it much easier for
other GCs to opt in.  However, this change does not alter the set of GCs
providing support; currently only G1 and Shenandoah support string
deduplication.  Adding support by other GCs will be in followup RFEs.

Reviewing via the diffs might not be all that useful for some parts, as
several files have been essentially completely replaced, and a number of
files have been added or eliminated.  The full webrev might be a better
place to look.

The major changes are in gc/shared/stringdedup/* and in the supporting
collectors, but there are also some smaller changes in other places, most
notably classfile/{stringTable,javaClasses}.

This change is additionally labeled for review by core-libs, although there
are no source changes there.  This change injects a byte field of bits into
java.lang.String, using one of the previously unused padding bytes in that
class.  (There were two unused bytes, now only one.)

Testing:
mach5 tier1-2 with and without -XX:+UseStringDeduplication

Locally (linux-x64) ran all of the existing tests that use string
deduplication with both G1 and Shenandoah.  Note that
TestStringDeduplicationInterned.java is disabled for shenandoah, as it
currently fails, for reasons I haven't figured out but suspect are test
related.

- gc/stringdedup/   -- these used to be in gc/g1
- runtime/cds/SharedStringsDedup.java
- runtime/cds/appcds/cacheObject/DifferentHeapSizes.java
- runtime/cds/appcds/sharedStrings/*

shenandoah-only:
- gc/shenandoah/jvmti/TestHeapDump.java
- gc/shenandoah/TestStringDedup.java
- gc/shenandoah/TestStringDedupStress.java

Performance tested baseline, baseline + stringdedup, new with stringdedup,
finding no significant differences.

-------------

Commit messages:
 - New string deduplication

Changes: https://git.openjdk.java.net/jdk/pull/3662/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3662&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8254598
  Stats: 6185 lines in 105 files changed: 2361 ins; 3202 del; 622 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3662.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3662/head:pull/3662

PR: https://git.openjdk.java.net/jdk/pull/3662