[YUNIKORN-1802] Add node events

[pbacsko]

What is this PR for?

Create wrapper type for node events and integrate it with the existing codebase.

What type of PR is it?

• - Bug Fix
• - Improvement
• - Feature
• - Documentation
• - Hot Fix
• - Refactoring

Todos

• - Task

What is the Jira issue?

https://issues.apache.org/jira/browse/YUNIKORN-1802

How should this be tested?

Screenshots (if appropriate)

Questions:

• - The licenses files need update.
• - There is breaking changes for older versions.
• - It needs documentation.

[codecov]

Codecov Report

Merging #573 (e17e434) into master (9bc3a5e) will increase coverage by 0.19%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #573      +/-   ##
==========================================
+ Coverage   76.41%   76.61%   +0.19%     
==========================================
  Files          74       75       +1     
  Lines       12212    12299      +87     
==========================================
+ Hits         9332     9423      +91     
+ Misses       2569     2565       -4     
  Partials      311      311              

Impacted Files	Coverage Δ
pkg/scheduler/context.go	30.63% <100.00%> (+0.20%)	⬆️
pkg/scheduler/objects/application_events.go	100.00% <100.00%> (ø)
pkg/scheduler/objects/node.go	84.16% <100.00%> (+1.56%)	⬆️
pkg/scheduler/objects/node_events.go	100.00% <100.00%> (ø)

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[pbacsko]

cc @FrankYang0529 you might need some stuff from this PR, like MockEventSystem. If this gets merged, you can rebase your PR on this. Let's make sure that our changes (methods, test cases, approaches, etc.) look similar for consistency.

[wilfred-s]

PR needs a rebase after other event changes have been committed.

[wilfred-s]

one minor point on locking