RFR: JDK-8170945: Collectors$Partition should override more Map methods

[Viktor Klang]

Adds overrides for common Map operations to avoid having to allocate the entrySet for Collectors$Partition maps.

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Commit messages:
 - Overriding containsKey and containsValue of Collectors
 - JDK-8170945: override more Map-methods on Collectors

Changes: https://git.openjdk.org/jdk/pull/13347/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13347&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8170945
  Stats: 29 lines in 1 file changed: 29 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/13347.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13347/head:pull/13347

PR: https://git.openjdk.org/jdk/pull/13347