proposer-duty: duty-fetch improvements

[iurii-ssv]

For Proposer-duty, we fetch duties on the very 1st slot of the current epoch (or on the slot that comes after the slot SSV node started at)

• if we have a Proposer-duty to execute in that same slot of that epoch we'll delay it for longer than necessary waiting for that fetch to happen (for attestations we fetch duties ~half an epoch ahead of time, for example)
• if we are not able to fetch the Proposer-duties from CL in some epoch - there will be no retrying of that fetch for that epoch (we'll just log the failed to fetch duties for current epoch error and won't execute any duties in that epoch)