Update used bootstrap as they become available + require trusted-block-root only at first start-up

[kdeme]

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.