[5.x]: Enable the use of a persisted FMRC cache

[lesserwhirls]

Description of Changes

In this PR, I've added hooks to allow for the runtime loading of a persisted FMRC cache (specifically, the GridDatasetInv objects). This is very much needed by the TDS. This does not need to be ported forward at this point. Unrelated, I also fixed two minor javadoc warnings. Closes #771.

PR Checklist

• Indicate the version associated with this PR in the Title
  (e.g. "[5.x]: This is my PR title")
• Link to any issues that the PR addresses
• Add labels, especially if the PR should be ported to other versions
  (these labels start with "port: ")
• Open as a draft PR
  until ready for review
• Make sure GitHub tests pass
• Mark PR as "Ready for Review"

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This change is 

[haileyajohnson]

Reviewed 3 of 4 files at r1.
Reviewable status: 3 of 4 files reviewed, 1 unresolved discussion (waiting on @lesserwhirls)

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cdm/core/src/main/java/ucar/nc2/ft/fmrc/GridDatasetInv.java, line 213 at r1 (raw file):

    return lastModified.getTime();
  }

are we planning on using version and REQ_VERSION outside this class?

[lesserwhirls]

Reviewable status: 3 of 4 files reviewed, 1 unresolved discussion (waiting on @haileyajohnson)

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cdm/core/src/main/java/ucar/nc2/ft/fmrc/GridDatasetInv.java, line 213 at r1 (raw file):

Previously, haileyajohnson wrote…

are we planning on using version and REQ_VERSION outside this class?

I have code in the TDS ready to go in a PR that uses both of those. The inventory cache implementation checks to see if the version of the inventory object read from the cache is compatible with the version of netCDF-java in use. If it's not, the old version is purged from the cache. Right now, those two values are only absolutely needed for log messages that I produce. I could refactor to get rid of exposing them, I think. Let me see.

[lesserwhirls]

Reviewable status: 3 of 4 files reviewed, 1 unresolved discussion (waiting on @haileyajohnson)

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cdm/core/src/main/java/ucar/nc2/ft/fmrc/GridDatasetInv.java, line 213 at r1 (raw file):

Previously, lesserwhirls (Sean Arms) wrote…

I have code in the TDS ready to go in a PR that uses both of those. The inventory cache implementation checks to see if the version of the inventory object read from the cache is compatible with the version of netCDF-java in use. If it's not, the old version is purged from the cache. Right now, those two values are only absolutely needed for log messages that I produce. I could refactor to get rid of exposing them, I think. Let me see.

Done. How does that look?

[haileyajohnson]

Reviewed 1 of 4 files at r1, 1 of 1 files at r2.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @haileyajohnson)

[haileyajohnson]

awesome, I'll merge when the tests finish!