RFR: 8287168: (fs) WatchService leaves thread running if not closed

[Brian Burkhalter]

Add a `Cleaner` to each `WatchService` implementation.

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Commit messages:
 - 8287168: (fs) WatchService leaves thread running if not closed

Changes: https://git.openjdk.java.net/jdk/pull/8986/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8986&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8287168
  Stats: 124 lines in 3 files changed: 102 ins; 18 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8986.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8986/head:pull/8986

PR: https://git.openjdk.java.net/jdk/pull/8986