RFR: 8261036: Reduce classes loaded by CleanerFactory initialization [v3]

[Claes Redestad]

> This patch refactors CleanerFactory and InnocuousThread so that we need to load fewer PrivilegedActions. This slightly reduce the number of classes we always load on bootstrap, slightly reducing static and dynamic footprint (default CDS archive -4Kb).

Claes Redestad has updated the pull request incrementally with one additional commit since the last revision:

  Only set priority if requested

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/2380/files
  - new: https://git.openjdk.java.net/jdk/pull/2380/files/043067eb..89a16eb6

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=2380&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=2380&range=01-02

  Stats: 3 lines in 1 file changed: 0 ins; 0 del; 3 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2380.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2380/head:pull/2380

PR: https://git.openjdk.java.net/jdk/pull/2380