ensure sync duties for next epoch are registered in time

[etan-status]

For attestations, VC queries duties for current and next epoch. For sync messages, VC queries for current and next period (if soon). This means that for sync messages we don't actually have the duties for next epoch in all situations, leading to situation where VC may miss sync duties in the final slot of an epoch when using. As duties remain same within a sync committee period, simply copy them over to next epoch to avoid this situation without adding network latency.

[github-actions]

Unit Test Results

         9 files  ±0    1 071 suites  ±0   39m 27s ⏱️ + 3m 45s
  3 698 tests ±0    3 419 ✔️ ±0  279 💤 ±0  0 ❌ ±0 
15 760 runs  ±0  15 455 ✔️ ±0  305 💤 ±0  0 ❌ ±0 

Results for commit 058b5d3. ± Comparison against base commit a6f0a7a.

[arnetheduck]

It does feel like the duties should really be keyed by sync committee period + dependent root here here rather than epoch - this would resolve this problem and avoid the extreme strain on the CPU due to the extra division potentially introduced in the future if period ratios become non-constant.