Change Monitor stat history to sync.Map

[rob05c]

What does this PR do?

Based on #3000 - recommend merging that first.

Change Monitor stat history to sync.Map

This significantly improves performance. In my testing, for a large
CDN, processing Stat results can take ~5 seconds, where ~1s of that
was copying and setting the mutexed stat history variable. This
essentially eliminates that 1s or 1/5 of stat processing time, for
every stat process. This significantly reduces CPU, and/or makes
the monitor able to poll stats in significantly less time.

Also changes the stat history array to not reallocate memory,
but move the data when prepending a new result. Which is also
significantly faster, in my testing.

Which TC components are affected by this PR?

• Documentation
• Grove
• Traffic Analytics
• Traffic Monitor
• Traffic Ops
• Traffic Ops ORT
• Traffic Portal
• Traffic Router
• Traffic Stats
• Traffic Vault
• Other _________

What is the best way to verify this PR?

Run Monitor, verify health and stats are polled correctly. This PR does not change the interface; it's an internal performance improvement.

Check all that apply

• This PR includes tests
• This PR includes documentation updates
• This PR includes an update to CHANGELOG.md
• This PR includes all required license headers
• This PR includes a database migration (ensure that migration sequence is correct)
• This PR fixes a serious security flaw. Read more: www.apache.org/security

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[dneuman64]

I know @ezelkow1 took a look at the code changes here, but I think this PR would benefit from a test or two.

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