DAL/Node: do not get plugins for old levels at startup

What

When a node starts for the first time, we do not expect it to store skip list cells (even if it supports refutations) for the entire required period of 3 months, because it will anyway not have the slot pages for this period.

This could be further refined later by taking into account when the corresponding rollup was originated.

Why

The DAL node imposes a too strong requirement on the L1 node's history mode.

How

Store the first seen level and then use it to determine the requirement on the L1 history and the period for which we need the plugins.