Support variable session length for Rococo chains at genesis (#2167)

This pr adds support to change the session length of a Rococo chain at
genesis. This is rather useful because Rococo has a session length of
1 hour, while on rococo-local you will now get 1 minute. This improves
the dev experience, because a parachain is only going live at the
start of a new session.

polkadot/Cargo.lock

@@ -5672,6 +5672,7 @@ dependencies = [
  "sp-offchain",
  "sp-runtime",
  "sp-session",
+ "sp-state-machine",
  "sp-storage",
  "sp-transaction-pool",
  "sp-trie",