Reduce number of active leaves at startup (#2631)

Currently we will take all leaves and give that to the overseer on
startup, but this is a bad idea when the finality is lagging for
example. There can be many of unfinalized leaves, we don't even need to
look at anymore. To solve this, the pr adds a maximum of 4 leaves we
forward to the overseer and the pr also checks that we only pass uncles
of the best block.