Caching ResourceManagerStringLocalizer creation

[sebastienros]

Fixes #1264

As per the issue details, the current code is doing too much reflection.
Here the _localizerCache is reused to add entries for the Type based initialization. The keys can't collide with the other usage of the dictionary.
AssemblyQualifiedName is used as the key to isolate all types in the dictionary.

[davidfowl]

@sebastienros Can you run a profile on this code so we can see the difference ?

[ryanbrandenburg]

Did we ever get our profile proof?

[JunTaoLuo]

Note that some sources edited in this PR has migrated to dotnet/aspnetcore. A new PR with the same changes for the affected sources should be opened in dotnet/aspnetcore. See #2974 for details.

[ghost]

As per aspnet/Announcements#411, we are currently migrating components from this repository to other repositories. This PR targets components that have been moved to dotnet/aspnetcore. If you're still interested in contributing this change, please retarget your PR to dotnet/aspnetcore and reference the original issue discussing the change or bug fix. If you have questions, or need guidance on how to port your change, please tag @dotnet/extensions-migration in a comment and we'll try to help.