SyntaxConstructClassifier cancels running previous async computation before starting a new one

fix after rebase
HighlightUsageTagger uses AllowStaleResults.No instead of MatchingSource
Merge pull request #502 from vasily-kirichenko/fix-highligh-refs-does-not-appers-after-renaming

HighlightUsageTagger uses AllowStaleResults.No instead of MatchingSource
Merge pull request #496 from vasily-kirichenko/fix-wrong-current-line-coloring

SyntaxConstructClassifier cancels running previous async computation before starting a new one