Make it sync parachains

[tomaka]

• [x] Make it sync Rococo.
• [ ] Make it sync Tick (or Trick or Track)

[tomaka]

Overview of how parachains work:

• On the parachain, collator nodes produce blocks and, in addition to gossiping them around, send them to the relay chain validator nodes, where they are verified. At the moment there is no restriction on who can actually produce blocks on the parachain. In other words it is possible for hundreds of nodes to produce blocks and create hundreds of forks.
• The relay chain then stores, in its storage, the best block (according to the relay chain validators) of the parachain. The persisted_validation_data runtime entry point can be called on the relay chain in order to obtain the parachain heads. See this.
• The current best block of the parachain is equal to the head stored in the current best block of the relay chain.
• The latest finalized block of the parachain is equal to the head stored in the latest finalized block of the relay chain.

In terms of changes to make in substrate-lite:

• [x] Finish the networking to make it possible to connect to multiple chains at once.
• [x] (for light clients) Implement the RemoteCallRequest light client request, in order to be able to call persisted_validation_data on the relay chain without having to store the relay chain storage.
• [x] Add a "FreeForAll" consensus algorithm where the block headers check always passes (and thus all blocks are accepted assuming they execute correctly). We might however still have to check the inherents, something that is still unclear to me at the moment, cc #227.
• [x] Add a "RelayChain"/"Manual"/"External" finality algorithm.
• [x] Add a new parachains syncing system in sync that emits requests for both the relay chain and the parachain, and that is responsible for calling persisted_validation_data. How to organize the code in the sync module is however completely unclear at the moment, as the code is presently very messy.