[signer] request for upcoming reward set can introduce 5s of delay

[jcnelson]

When signing blocks in a prepare phase, the anchor block for the upcoming cycle is requested by the signer in order to learn the set of next signing keys. However, the anchor block may not yet exist. The absence of the anchor block leads to the signer spending 5 seconds waiting for data that isn't available in the first place before it proceeds to handle StackerDB messages.