Deadlock in streaming connected components

[ehein6]

Streaming connected components can deadlock when handling edge deletions. The function update_tree_for_delete_directed() applies a bitwise not to a value of the level array.

If the level value was zero, it will be flipped to -1. This is a reserved value for the full/empty bit functions that means "empty". Later, when bfs_rebuild_component() attempts a readff() on the same value, it hangs. This occurs even during serial execution.

Somehow we need to prevent a -1 from being ever being written to a level value, but it's not clear how to do this without breaking something else.

[ehein6]

Something is wrong here, see #234. Starting from an empty graph, we add (1,2), (2,3), (3, 4), then remove (2,3). The code still thinks everything is in the same component.