[Optimisation] Avoid reprocessing of locally produced blocks

[tbenr]

Description

We could avoid reprocessing blocks that has been locally generated\proposed.

[ajsutton]

The EL will already optimise this so I wouldn't try to special case it. If we were going to special case it, we'd be better off skipping the processing of the whole block - we'd have to check the block signature since that's created by the validator but could just keep a short term cache of the BeaconBlock and resulting BeaconState to import without reprocessing.

[lucassaldanha]

Closed as not planned.