Only check operation merkle root once per block

Context

Whenever a block was received from a peer, we would check that its operations' merkle root are consistent for every peer while the block was being validated/applied. This MR moves the check later in the control-flow so that we first check if the block is already known or currently being treated before doing this expensive computation.

Manually testing the MR

CI

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR