Refactor OCW processing

[stiiifff]

No description provided.

[jimmychu0807]

I think overall this is better than the previous(my) primitive implementation:

• using offchain storage to store up to which block of events we have processed
• processing http request events in a batch from previously processed block all the way to the current block
  👍

We can merge this to master first. One thing I want to improve on is about the locking. Recent Substrate has implemented Storage Lock, so we can just use the lock instead of using a storage value to simulate the locking mechanism. Do you want to fix it in this PR altogether, or leave it as a todo for next iteration?

(todo: I know I need to update the recipe to use showcase this storage lock instead of using the storage value to simulate the locking mechanism 😄 )

[riusricardo]

LGTM and agree that we should change the lock type.
I would merge this, the lock change isn't a blocker.