It now takes a `proto_header` in parameter, and it returns a full
`shell_header`. This prepares the inclusion of the context's hash in the
`shell_header`.
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
