RFR: 8266637: CHT: Add insert_and_get method

[Ivan Walulya]

Hi all,

Please review this change to add an `insert_get` method to the CHT.  The method combines `insert()` and `get()` methods to decrease the amount of global synchronization needed for `get()` calls after an `insert` call. 

Testing: tier1-3

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Commit messages:
 - fix memory leak
 - change comment
 - change comment
 - add insert_get method and gtest

Changes: https://git.openjdk.java.net/jdk/pull/3915/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3915&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8266637
  Stats: 38 lines in 3 files changed: 30 ins; 0 del; 8 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3915.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3915/head:pull/3915

PR: https://git.openjdk.java.net/jdk/pull/3915