Transactions dependency graph (#787)

* Graph transaction pool.

* Start future implementation.

* Future -> Ready promotions.

* Replacement logic.

* Clear extern crates, add docs.

* Move hash externally.

* Implement remove_invalid

* Implement ready transactions pruning.

* Move & rename.

* Add some logs.

* Clean up deps.

* Use Member trait.

* Add missing docs, elaborate on the proof.

* Expand on docs and proofs.