Concurent request of the same ressource might insert multiple 'Pending
request' in the request tracking table. Resulting, only one of them
will ever be satisfied and some worker might be stuck for ever. We
avoid this be removing any cooperation between lookup and insertion in
the table.
Let's get serious. The full index of operations is not sustainable in
the production code. We now only keep the index of operations not yet
in the chain (i.e. the mempool/prevalidation). Operations from the
chain are now only accesible through a block. For instance, see the
RPC:
/blocks/<hash>/proto/operations
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.
