RFR: 8278602: CDS dynamic dump may access unloaded classes

[Ioi Lam]

Cause of crash:

When dumping a CDS archive,  while iterating over entries of the `SystemDictionaryShared::_dumptime_table`, we do not check whether the classes are already unloaded. In the crash, we are trying to call `InstanceKlass::signer()` but the class has already been unloaded.

Fix:

Override the  template function `DumpTimeSharedClassTable::iterate` to ensure iteration safety.  Do not iterate over a class if its `class_loader_data` is no longer alive.

The assert in `DumpTimeSharedClassTable::IterationHelper` found another existing bug -- we were calling `SystemDictionaryShared::is_dumptime_table_empty()` without holding the `DumpTimeTable_lock`. I delayed the call until we have grabbed the lock.

Testing:

I have attached a test case into the bug report. Without the fix, it would reproduce the same crash in less than a minute. With the fix, the crash is no longer reproducible.

Unfortunately, the test case requires a ZGC patch (thanks to @stefank) that adds delays to  increase the likelihood of seeing unloaded classes inside the `_dumptime_table`. Therefore, I cannot integrate the test as a jtreg test. I'll mark the bug as **noreg-hard**

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Commit messages:
 - cleaned up code
 - add #if INCLUDE_CDS
 - Merge branch 'master' into 8278602-cds-zgc-class-unload-bug
 - using k->is_loader_alive() is enough
 - Added DumpTimeSharedClassTable::iterate() to make sure every iteration goes through EligibleClassIterationHelper
 - step1

Changes: https://git.openjdk.java.net/jdk/pull/6859/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6859&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8278602
  Stats: 104 lines in 6 files changed: 96 ins; 4 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6859.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6859/head:pull/6859

PR: https://git.openjdk.java.net/jdk/pull/6859