cubicweb #506805 add cube instance version string to dump filename [open]

I would change the default format of dump file by adding the current instance version.

So, It can help when you have a lot of files in backup directory.

For example, we could add info as a prefix to facilitate shell listing:

VERSION:%Y-%m-%d_%H-%M-%S
priorityminor
typeenhancement
done in<not specified>
load0.100
load left0.100
closed by<not specified>