Improve `prepare_hived_data_5m` and similar
Atm to perform 5M tests we are copying 5M block_log file from source location provided by runner to the local directory used by job producing data.
To reduce server resource usage, let's change a regular block_log copy to creation of hardlink, so statement similar to this one: ln <source> <target_block_log>
Then, while performing a whole data directory copies we should add a --link
option to the cp
command to preserve earlier created hardlinks.