RFR: 8257602: Introduce JFR Event Throttling and new jdk.ObjectAllocationSample event (enabled by default)

[Markus Grönlund]

Greetings,

please help review this enhancement to let JFR sample object allocations by default.

A description is provided in the JIRA issue.

Thanks
Markus

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Commit messages:
 - defensive initialization check
 - Whitespace errors
 - JFR Event Throttling

Changes: https://git.openjdk.java.net/jdk/pull/1624/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1624&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8257602
  Stats: 2607 lines in 43 files changed: 2346 ins; 238 del; 23 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1624.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1624/head:pull/1624

PR: https://git.openjdk.java.net/jdk/pull/1624