Add benchmarks to pallet propagation

[JesseAbram]

Related #1009

[ameba23]

Looks good to me - so if i understand right, even though the propagation pallet has no extrinsics, we still need a weight for the on_initialize hook, as it makes writes to runtime storage.

Approving pending fixing CI - looks like a trivial fix.

[JesseAbram]

Looks good to me - so if i understand right, even though the propagation pallet has no extrinsics, we still need a weight for the on_initialize hook, as it makes writes to runtime storage.

Approving pending fixing CI - looks like a trivial fix.

yes but not runtime storage, pretty much every operation that takes any form of time should be benchmarked (even if it doesn't require a fee) because it informs the chain how much time the operation takes up in a block, as block space is finite