Benchmark pallet session

We need to fork the crate pallet-session-benchmarking to replace pallet staking by our autority-members pallet.

changed milestone to %runtime-400

added C-runtime D-medium P5-sometimesoon S-ToDo labels

mentioned in issue #81 (closed)

changed milestone to %runtime-500

added ET-weeks Mandatory-prod labels

mentioned in merge request !156 (merged)

assigned to @bgallois

Done with !156 (merged).

closed