This refactors `src/node/shell/state.ml` in order to trace the source of
blocks and operations. This prepares the node for the three-pass
validator.
In the procces, it adds an in-memory overlay for blocks and operations.
* fix bad "locking" in `p2p_io_scheduler.shutdown`
* add missing exception handler in `P2p_io_scheduler`
* wake-up `P2p.recv_any` when a new connection is established
* reintroduce version negotiation
* fix typo in greylisting