pallet-ethereum: kill Pending storage when finalizing block

[koushiro]

Signed-off-by: koushiro koushiro.cqx@gmail.com

The current implementation will store the Pending on-chain, but I don't think it's necessary

[koushiro]

/cc @tgmichel @AsceticBear

[boundless-forest]

LGTM, the pending data has been stored in store_block(...).

[tgmichel]

cc @sorpaas This PR introduces a bug in pallet-ethereum storage on runtime upgrade.

Block #1 - Runtime version 1
- on_initialize kill Pending -> Storage Pending []
- User sends transaction A
- Storage Pending[A]

Block #2 - Runtime version 1
- on_initialize kill Pending -> Storage Pending []
- User sends transaction B
- Storage Pending[B]

---- Runtime Upgrade version 1 -> version 2 -----

Block #3 - Runtime version 2
- (on_initialize kill Pending WAS REMOVED)
- User sends transaction C
- Storage Pending[B, C]
- on_finalize kill Pending -> Storage Pending []

Results in transaction B to exist in runtime storage for two different blocks (and mapped in the offchain db also).

[tgmichel]

So a migration is needed.