[13] RFR(L): 8214542: JFR: Old Object Sample event slow on a deep heap in debug builds

[Markus Gronlund]

Greetings, 

Kindly asking for reviews for the following changeset:

Bug: https://bugs.openjdk.java.net/browse/JDK-8214542 
Webrev: http://cr.openjdk.java.net/~mgronlun/8214542/webrev01/ 
Testing: test/jdk/:jdk_jfr

Summary:
Removal of heavy current_frontier assertion.
Trimmed down storage requirements by only storing necessary contextual edges.
Normalized representation of edges for better reuse.
Better preservation of already persisted tree structure by less intrusive reuse logic.
Caching.

Thank you in advance
Markus