It seems #5246 introduced a subtle bug that lead to the error "out of
order block: expecting height=1, got height=XXX" some times during
startup. Apparently it can happen that during pruning of the graph tip
some blocks can come in before we start our chain view and the new block
subscription. By querying the chain backend for the best height before
syncing with the graph we ensure that we never miss a block.
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