Add "Unfavorite" edge indexing into tweet counting algorithms

[guimingTang]

This request adds the functionality to "remove" tweet favorite edges that are later on unfavorited. For example, userA "favs" and later on "unfavs" Tweet1. This has been a much requested feature, but inherently, GraphJet's internal graph storage is read-only, making it impossible to undo a favorite edge.

One alternative to achieving this feature, at least from the client's point of view, is that we introduce a counter edge, "unfavorite" edge, and index that edge into the graph. During graph traversal, for each tweet, we collect every engagement edge, including fav and unfavs, into a NodeInfo object. Before ranking, however, we re-check the NodeInfo and look for possible fav-unfav edge pairs. We remove these pairs from NodeInfo in-place should they exist. This way the unfavorited edge will be absent during ranking, and thus not returned to the client.

[CLAassistant]

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