Consider having Provided solely block the scanner, not the Tributary

[kayabaNerve]

Provided halting the Tributary was a safety feature, yet it has the DoS of preventing the 33% who have yet to receive the Provided Transaction from participating in consensus. By having verify_block still reject it, yet add_block handle it, we prevent this DoS.

There'd still be the guarantee 67% have come to sync on the item, and the scanner still waiting for the Provided transaction to be locally provided will ensure safety.

This should reduce risk of stutters in Tendermint.

[kayabaNerve]

https://github.com/serai-dex/serai/actions/runs/6238640448/job/16934913654 failed, likely due to a proposer who was late on a Provided failing to propose, adding several seconds of latency to the run.

[kayabaNerve]

To clarify, a proposer late on a Provided did fail to propose. The "likely" is referring to that being THE reason the run failed. Not due to uncertainty about that event occurring.

[kayabaNerve]

Fixed by #381.