Stop keeping track of destroyed node-to-node channels

[jumaffre]

We currently keep track of destroyed node-to-node channels (i.e. replace the std::shared_ptr<Channel> with nullptr in the channels map, instead of simply deleting the NodeId key).

We should try to fix, considering what may potentially break in some end-to-end tests (e.g. code update, reconfiguration).

[achamayou]

Link #2672

[eddyashton]

This was done as part of #2801. In moving NodeToNodeChannelManager to its own file, we also removed the destroy_channel paths which filled the map with tombstone null pointers.