chifra init: downloading timestamp files

[tjayrush]

Once we have the ability to add 'reason' to the smart contract, we can add records for "timestamp". In that case, when user does chifra init or chifra init --all they can automatically also download the timestamp database.

We have to be a bit careful here, though.

We can download the ts.bin file to a temporary file and compare it's size to the existing ts.bin. Only replace the existing if it's smaller (i.e., it further along than the existing file).