Add experimental cache self-monitoring

[rob05c]

Adds an experimental Proof-of-Concept project to replace the Cache Health Monitoring part of Traffic Monitor with cache remap rules, so the caches monitor themselves.

This has a number of benefits.

• saves TC operators having to deploy a CPU- and network-intensive application.
• saves the TC project the difficult and expensive development of a high-performance application.
• Automatically horizontally scalable, solves the current monitor scalability problem out of the box.

In a nutshell, there are 3 components:

• astatstwo replaces astats, monitors health on the cache, and returns a 204 or 503.
• remapgen generates remap rules for any cache to forward health requests to any other cache.
  • will include remaps for near, far, IPv4, and IPv6 health.
• healthcombiner requests the health of all caches from localhost and builds /CRStates.json consumed by Traffic Router.
  • Not strictly necessary, the Router could request health for each cache, but this allows us to avoid modifying the Router.

See the README.md for more details.

No tests, no docs, no changelog, experimental PoC.

• This PR is not related to any other Issue

Which Traffic Control components are affected by this PR?

None, experimental.

What is the best way to verify this PR?

Follow the instructions in the README.md to create the experimental setup.

If this is a bug fix, what versions of Traffic Control are affected?

Not a bug fix.

The following criteria are ALL met by this PR

• This PR includes tests OR I have explained why tests are unnecessary
• This PR includes documentation OR I have explained why documentation is unnecessary
• This PR includes an update to CHANGELOG.md OR such an update is not necessary
• This PR includes any and all required license headers
• This PR ensures that database migration sequence is correct OR this PR does not include a database migration
• This PR DOES NOT FIX A SERIOUS SECURITY VULNERABILITY (see the Apache Software Foundation's security guidelines for details)

Additional Information