Signer/Aggregator nodes sign only when Cardano node is synchronized

[jpraynaud]

Issue

The signer and aggregator nodes should not start signing and gathering stake distribution until the underlying Cardano node is (almost) fully synced

[jpraynaud]

Since we have implemented the era mechanism, the behavior of the aggregator node at startup has changed and partially mitigated this issue: the era reader reads an Utxo on the chain which is not available, which makes the node panic very early.

This issue is closely related to the #945