#2965 Atomic ZNode creation on node registration

[jacob-netguardians]

Issues

• My PR addresses the following Helix issues and references them in the PR description:

Fixes #2965

Description

• Here are some details about my PR, including screenshots of any UI changes:

If two nodes try to register at about the same time in the cluster, there may be a race condition while effectively adding the instances to the cluster description in zookeeper (the second one seeing an incomplete node hierarchy and later on refusing to accept tasks). Having an atomic creation of those nodes in zookeeper should prevent that.

Tests

• The following tests are written for this issue:

No additional tests.

Commits

• My commits all reference appropriate Apache Helix GitHub issues in their subject lines. In addition, my commits follow the guidelines from "How to write a good git commit message":
  1. Subject is separated from body by a blank line
  2. Subject is limited to 50 characters (not including Jira issue reference)
  3. Subject does not end with a period
  4. Subject uses the imperative mood ("add", not "adding")
  5. Body wraps at 72 characters
  6. Body explains "what" and "why", not "how"

Code Quality

• My diff has been formatted using helix-style.xml
  (helix-style-intellij.xml if IntelliJ IDE is used)