When the network is partitioned for a long time, e.g., Goerli, branches
start forming where different peers have distinct views about the chain
state. The current syncing solution with sync manager doesn't handle the
case well, as it is optimized for a healthy network where syncing can be
parallelized across different peers. To support sync manager discovering
additional branches, a new module is added that pulls in histories from
peers on unknown branches in a backwards manner.
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