Allow different ways of limitting the transactions in a block

[tomusdrw]

Currently we limit the size of all extrinsics in the block (see #1581) and have the limit hardcoded in the runtime.

However interaction with other modules (namely srml-contract) may require some other mechanism of limitting the number of transactions. For instance contracts use up gas and we can have a block gas limit alike Ethereum.

Would be perfect if the weight of the transaction was also given to the pool to allow better (more optimal) allocation of extrinsics to block to maximise both: fee and utilisation

It's a follow up issue on #1354 (see comment #1354 (comment))

[rphmeier]

When we're building blocks we also want to be informed of when this limit has been hit and how close we are to it. If the proposer can get back an Error::BlockFull(remaining_weight) and finish proposing / push more transactions withsum(new_txs.weight) <= remaining_weight that would work really well.

[tomusdrw]

I guess that's resolved with #3157