Relax fast-sync-min-peers

[garyschulte]

Description

Requiring a minimum of 5 peers before beginning a fast-sync has been deemed too costly, especially on low participation networks. Similarly, post-merge there is no need for a peer minimum at all, since the consensus layer directs besu to the exact hash that should be used for the pivot block for a fast-sync via the finalized head.

This PR suggests we should relax the minimum peer requirement for starting a fast sync to 2, and removing this requirement entirely post-merge.

Acceptance Criteria

• PoW, PoA networks can fast-sync with a minimum of 2 peers
• Post-merge networks can fast-sync immediately, as long as they have peers that satisfy the pivot/finalized block requirements coming from the consensus client

[macfarla]

fast-sync-min-peers is used by FAST and SNAP sync

[macfarla]

@garyschulte can we close this ticket since the PR is merged? Or is there still work to be done?

[garyschulte]

Let's close it.