Rework TxPool dependency graph to avoid crashes and strange behaviours

[xgreenx]

Overview

TxPool transactions management logic is complex and smeared across several files, leading to inconsistent updates.

As an example, it is possible to cause a crash on the line below:

Because the remove function missed some dependent transactions from the by_dependency field.

Solution

We need to rework the transaction pool and incorporate the logic of the transaction management side of one structure. Taking into account the work required for #1229 and #868, it seems we can simplify the logic of how transactions are validated. We don't need to replace transactions based on tips. We just can keep them in memory in another dependency graph.

More simple rules should reduce room for error.