Update 'need_bootstrappers' to have dinamically threshold

[herr-seppia]

Summary

This issue aims to enhance the 'need_bootstrappers' function by updating it to use 'alive_nodes' instead of 'closest_peers'. The current implementation introduces a hardcoded threshold value of 10 during the bootstrapping procedure, which deviates from the specified recommendation of 3 in the protocol specification.

Possible solution design or implementation

To address this issue, the 'need_bootstrappers' function should be modified to utilize 'alive_nodes' for determining the minimum number of nodes during the bootstrapping phase. This change ensures that the threshold is no longer hardcoded, making it configurable by the library user.

Additional context

By allowing users to set their own threshold, we empower them to tailor the bootstrapping procedure to their specific needs.
Additionally, the contact rate should be customizable too (now is 30secs)