This commits introduces support in DepGraph for the transaction positions to be
non-continuous. Specifically, it adds:
* DepGraph::RemoveTransactions which removes 0 or more positions from a DepGraph.
* DepGraph::Positions() to get a set of which positions are in use.
* DepGraph::PositionRange() to get the highest used position in a DepGraph + 1.
In addition, it extends the DepGraphFormatter format to support holes in a
compatible way (it serializes non-holey DepGraphs identically to the old code,
and deserializes them the same way)
