It wait for the node to be synchronized with the network. The heuristic
is currently:
- the timestamp of current head is less than 1 minute old ;
- there was a period of 30 seconds without new block discovered.
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.
