BREAKING: Static field cleanup, #662, #679

[paulirwin]

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Converts static readonly fields to const, where possible. Also addresses csharpsquid:S2223 issues with mutable static fields.

Fixes #662
Fixes #679

Description

This PR converts most static readonly fields to const where it is possible to do so. Some notable exceptions:

1. The analysis was not focused on any fields in unit test projects.
2. Any fields whose values changed in the latest Lucene were kept as static readonly, to be on the safe side.
3. If there were other, similar fields that needed to be static readonly, the ones that were const-able were left as-is for consistency.
4. Fields like VERSION_CURRENT were left as-is, since it might be expected to evaluate those.

This also addresses #679 with non-readonly static fields, where one of three things happened:

1. If the visibility should be private, this visibility was changed to private.
2. If the field really should have been readonly or const, this was changed appropriately.
3. If the field really should be mutable, it was changed into a static property.

[paulirwin]

Converted to draft; I'm going to include the related #679 changes as well.

Update: this is complete.

[paulirwin]

@NightOwl888 I'm iffy on ExtractWikipedia.Count. It's clearly internally mutable, and in Lucene it's possible to mutate it externally, but I don't think it's really supposed to be externally mutable (or even externally readable for that matter). It's pretty low priority to fix this, as this is just in benchmark code so not likely used in the real world, but let me know what you would do here. I could either make it have a private setter, or make it just a private static field instead of public. I also didn't change the increment to use Interlocked since this is not likely to be called so often in a multi-threaded way to where that would be an issue, but I could.