Don't propose/attest blocks during sync

[pablodeymo]

In the sync process, we don't have the full picture to propose or attest blocks. We should determine if we are still syncing in some way, and skip block proposal/attestation duties until we catched up.

Tip: check how other lean and beacon chain clients do this

[thinktanktom]

Hi, I'd like to work on this! I'll send a PR shortly.