Use "local" SD in Mithril Aggregator

[jpraynaud]

The stake distribution must be retrieved from a local store that will be fed with data from a Cardano node (feeding will be done separately):

• Create a stake_store module that reads/writes stakes by epoch
• Wire the stake_store in the aggregator
• Implement 'get_stake_distributionandupdate_stake_distributionwith this store in themulti_signer`
• Remove any reference to the stakes in the pending certificate