Improve beacon chain explorer

[Neylix]

Description

Beacon explorer subscribe to other node and itself to be notified when a new replication attestation is emitted.
But the node is notified for each confirmation while it only needs one confirmation.
Beacon node also notify storage node for a transaction, storages nodes already know the transaction
Beacon explore ask for notification to a node even if it is already subscribed.
All these actions spam a lot the node and they can get overloaded.

This PR solves these issue.

Fixes part of #425

Type of change

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

How Has This Been Tested?

Add some latency in connection.ex (Process.sleep(200)), run 3 nodes, open a beacon explorer and you should not have a lot of timeout after some time

Also added test files

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