txgraph: Add initial version (feature)

This adds an initial version of the txgraph module, with the TxGraph class.
It encapsulates knowledge about the fees, sizes, and dependencies between all
mempool transactions, but nothing else.

In particular, it lacks knowledge about txids, inputs, outputs, CTransactions,
... and so forth. Instead, it exposes a generic TxGraph::Ref type to reference
nodes in the TxGraph, which can be passed around and stored by layers on top.

Author: sipa

src/CMakeLists.txt

@@ -301,6 +301,7 @@ add_library(bitcoin_node STATIC EXCLUDE_FROM_ALL
   signet.cpp
   torcontrol.cpp
   txdb.cpp
+  txgraph.cpp
   txmempool.cpp
   txorphanage.cpp
   txrequest.cpp