refactor(share/pruner): make pruner init to start from 0

[walldiss]

Implementation ideas

Pruner starts with initial checkpoint with height 1, but still needs to prune that height on first run. It is managed by

if lastPruned.Height() == 1 {
		headers = append([]*header.ExtendedHeader{lastPruned}, headers...)
	}

Rework initial checkpoint and logic for first run to use 0 height at init.