clear FCS caches when a solution is opened
replace ProjectProvider.ReferencesChanged with a callback function
clear ProjectFactory cache on solution closing
VSLanguageService clears FCS caches on solution closing instead of opening, which caused VS freeze on loading large solutions
Merge pull request #477 from vasily-kirichenko/fix-memory-leakages

Fix memory leakages
translate saved current word span to the current snapshot in HighlightUsageTagger

*
SyntaxConstructClassifier unsubscribes from OnBuildProjConfigDoneEventHandler if doc.TextBuffer is null
remove redundant assertions
Dispose classifiers correctly
remove ITextBuffer from SyntaxConstructClassifierProvider dictionary after disposing
Merge pull request #479 from vasily-kirichenko/fix-wrong-current-word-snapshot

translate saved current word span to the current snapshot in HighlightUsageTagger