[EpochSync] Better generation of EpochSyncInfo::all_block_hashes

[posvyatokum]

In #10081 vector of all epoch block hashes is generated at the end of the epoch by iterating through BlockInfo column.
This may prove to be too big of a delay for block production.
Alternative solution is to keep list of block hashes in memory while keeping up with the network. This solution is not robust to restarts during the epoch, but it is still better than nothing.