Relaying tx/blocks by light clients

[svyatonik]

Brief overview:

• light clients are now not importing transactions coming from network;
• light clients are now not announcing imported blocks;
• extracted arguments of Service::new into separate trait Components with two implementations (FullComponents and LightComponents) => Service is now generic over Components (instead of Backend, CallExecutor and PolkadotApi).

Open question (not a part of this PR) - when/how to remove own transactions from the pool.

[gavofyork]

One strategy that comes to mind is to simply download all extrinsics for blocks following the submission of a transaction to check for inclusion.

This could be optimised later by:

• only storing the extrinsic hash in the extrinsic trie (thereby cutting down the download size); and
• monitoring the storage entry for the sender's transaction index, then only bothering to get the extrinsics when that changes.

Of course the former needs a protocol alteration and the latter requires light clients be able to efficiently monitor storage items in the state trie (see paritytech/polkadot#131).

[rphmeier]

Yeah, the way we do it in the Ethereum light client is just to check every few minutes what the current nonce of each account in the queue is and cull transactions based on that. We could apply a similar strategy for Polkadot, but there isn't a general solution for extrinsics just because they are a very flexible concept.