Sync in private net tries to request the DAO fork block

Reported in https://github.com/ethereum/aleth/issues/5263

When `daoHardforkBlock` is missing from config, it has some default non-zero value (`c_infiniteBlockNumer`) and the code in `BlockChainSync` requests DAO fork block when starting to sync from the new peer always when DAO fork block number is non-zero.

I think we should check in `BlockChainSync` for both 0 and `c_infiniteBlockNumer`