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

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SyntaxConstructClassifier unsubscribes from OnBuildProjConfigDoneEventHandler if doc.TextBuffer is null
remove redundant assertions
Dispose classifiers correctly
remove ITextBuffer from SyntaxConstructClassifierProvider dictionary after disposing