RFR: 8263903: Use Cleaner instead of finalize to auto stop Timer thread [v3]

[Kim Barrett]

> Please review this change to java.util.Timer, replacing the use of deprecated finalization-based cleanup with use of java.lang.ref.Cleaner.
> 
> In addition, Timer.cancel now cancels any later execution of the the no longer relevant cleanup.
> 
> Testing:
> mach5 tier1
> New AutoStop test verifies the specified cleanup behavior.
> (There are existing tests involving Timer.cancel.)

Kim Barrett has updated the pull request incrementally with one additional commit since the last revision:

  make ThreadReaper constructor non-public

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/3106/files
  - new: https://git.openjdk.java.net/jdk/pull/3106/files/2153a4c3..57500464

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=3106&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=3106&range=01-02

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3106.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3106/head:pull/3106

PR: https://git.openjdk.java.net/jdk/pull/3106