-v2: warn about const storage class

As const is qualifier in D2 there is no striaghtforward
replacement for this D1 pattern. Most simple approach is to make
those variables mutable for transitional period and re-qualify
later on case by case basis.
Merge pull request #4058 from mihails-strasuns-sociomantic/v2-const-storage-class

[D1] -v2: warn about const storage class