What

Check that the delegate is not forbidden when validating a consensus operation in mempool mode too.

Why

This avoids the mempool validating operations that can't be included in a valid block, which is bad for the baker. (Note: this could only happen when the forbidden delegate map was not empty, ie. when a delegate has committed at least two double signings, which usually doesn't happen.)
Preventing forbidden delegates' operations from being propagated through the network helps protect them from incurring further slashings related to operations that can't threaten the chain anyway because they can't be included in blocks, leaving them some time to react of the double signing was caused by a faulty configuration.

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