Update used bootstrap as they become available + require trusted-block-root only at first start-up
Currently when we start a fluffy node and want to enable the Portal beacon network, we need to provide a `trusted-block-root` and always start from that one.

We should make following  changes which would highly improve the UX:
- Require to provide `trusted-block-root` only on first start
- Or require to provide `trusted-block-root` when last stored bootstrap is not `is_within_weak_subjectivity_period`. (perhaps with potential overwrite flag on this)
- When a fluffy node is consensus light client synced, update the actually bootstrap starting point each time to the last received bootstrap.