Cache the walking of the CultureInfo tree in ResourceManagerStringLocalizer

[DamianEdwards]

Implemented caching of the walk of the CultureInfo tree to get resource names. Includes tests.
#15

@davidfowl @Eilon @muratg

[muratg]

Not sure what @davidfowl thinks, but in some of the projects, we moved Internal classes to an .Internal namespace and made them public for testability instead. We don't seem to be following this on all projects, so just something to think about.

[DamianEdwards]

In this case it's just an internal method to clear a global static cache, not types.

[muratg]

Looks good,

[hishamco]

@DamianEdwards for better organization, can we introduce an interface named IResourceCache that resource cache adapters have to implement

[DamianEdwards]

@hishamco This isn't something that should be abstracted. It's an implementation detail of this specific IStringLocalizer. It doesn't make sense to create a provider/adapter model here.

[DamianEdwards]

Merged in 9384848