handle multi-root dag in DescendantsFlow

[agaurav]

currently DescendantsFlow will wait indefinitely if any of the descendants nodes has a parent that's not in path from start node to itself as the walk function never reaches there. This will be always true when dag has multiple roots and one of them being a start node.

to fix that, this pr changes the size of inputChannels from all parents to only parents who are descendant of start node

[agaurav]

all tests are passing.

[agaurav]

@sebogh requesting review in case this is still actively maintained.