Operations now include a block hash in their header. Such an operation
could only be included in a successor of this block.
Furthermore, when validating a block, the economic protocol now
returns---together with the context---an integer `max_operations_ttl`.
Then, when validating a successor, the shell will fail if it contains
an operation whose header's block hash is not one the
`max_operations_ttl` predecessors of the block.
As a bonus, the shell is now able to detect and forbid replayed
operations. Then, we might decide to remove some replay
detection-mechanism that we previously implemented in the economic
protocol.
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`.
