Better FeatureCompatibility const correctness

I was surprised when

if (const auto& fcv = serverGlobalParams.featureCompatibility; fcv.isVersionInitialized() &&

    fcv.isGreaterThanOrEqualTo(ServerGlobalParams::FeatureCompatibility::Version::kVersion47)) {

    // ...

}

wouldn't compile. So the newly-added accessors (isLessThanOrEqualTo, isGreaterThanOrEqualTo, isLessThan, isGreaterThan, isUpgradingOrDowngrading) should be const-qualified.

However in addition to this it might be better if serverGlobalParams.featureCompatibility was a const ref, with some other name for the places that legitimately need to modify it, eg.

 struct ServerGlobalParams {

     // ...

     struct FeatureCompatibility {

         // ...

-    } featureCompatibility;

+    } featureCompatibilityMutable;

+    const FeatureCompatibility& featureCompatibility = featureCompatibilityMutable;

     // ...

 };

This would have caught the missing const-qualifier on the accessors when they were added (in addition to helping prevent accidents).