call InteractiveChecker.InvalidateAll after solution reloaded

add assertion to localize a bug with wrong symbol ranges
fixed: SourceCodeClassifier raises exception on indexers

recreate Checker for each solution
replace Atom holding LanguageService instance with a mutable field
cosmetics
Merge pull request #223 from vasily-kirichenko/invalidate-checker-on-solution-loading

call InteractiveChecker.InvalidateAll after solution reloaded