Improve fetch summaries timeout

[Neylix]

Description

in BeaconChain.fetch_and_aggregate_summaries there where 2 nested Task.async. This increase the risk of getting timeout.
To reduce this risk, the chunk by addresses is done before creating a task and there is only one task per {node, chunked addresses}

Fixes #1077

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

Unit test still passes, running multiple node with benchmark

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

[bchamagne]

Looks good to me, I only tested there are no regression:
I ran this on 3 nodes:

Archethic.BeaconChain.SummaryTimer.previous_summary(DateTime.utc_now()) |> Archethic.BeaconChain.fetch_and_aggregate_summaries(Archethic.P2P.authorized_and_available_nodes())