RFR: 8271506: Add ResourceHashtable support for deleting selected entries [v2]

[Coleen Phillimore]

> The ResourceHashtable doesn't have a way to delete selected entries based on things like their class has been unloaded, which is needed to replace Hashtable with ResourceHashtable.
> The Nodes of the ResourceHashtable has a Key and Value of  template types.
> template<typename K, typename V>
> class ResourceHashtableNode : public ResourceObj {
> public:
>   unsigned _hash;
>   K _key;
>   V _value;
> ...
> But there's no destructor so that ~K and ~V are not called (if I understand C++ correctly).
> 
> When instantiated with a value that's not a pointer, calling code does this:
> 
>   SourceObjInfo src_info(ref, read_only, follow_mode);
>   bool created;
>   SourceObjInfo* p = _src_obj_table.put_if_absent(src_obj, src_info, &created);
> 
> So if SourceObjInfo has a destructor, it'll have to have a careful assignment operator so that the value copied into the hashtable doesn't get deleted.
> 
> In this patch, I assign the responsibility of deleting the Key and Value of the hashtable to the do_entry function, because it's simple.  If we want to use more advanced unreadable C++ code, someone will have to suggest an alternate set of changes, because my C++ is not up to this.
> 
> Tested with tier1-3, gtest, and upcoming patch for JDK-8048190.

Coleen Phillimore has updated the pull request incrementally with one additional commit since the last revision:

  Make unlink non-const because it's not.

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/4938/files
  - new: https://git.openjdk.java.net/jdk/pull/4938/files/d3e23f7a..999405db

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=4938&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=4938&range=00-01

  Stats: 9 lines in 1 file changed: 5 ins; 1 del; 3 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4938.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4938/head:pull/4938

PR: https://git.openjdk.java.net/jdk/pull/4938