Save the last processed index of the certstream log when certstream-server-go exits

[messede-degod]

Saving the last processed index of the certstream log when certstream-server-go exits allows the user to continue where they left off and not miss out the previous certificates.

[d-Rickyy-b]

I'll definitely look into this. Currently the application doesn't store anything on the machine, so we'd need some kind of storage format. That could be as simple as a json file to not introduce new dependencies. What do you think?

[messede-degod]

yep, this sounds good, i think storing [certstream.Entry].Data.Source.URL and [certstream.Entry].Data.CertIndex in a map (similar to what we do for metrics) and persisting it should do it.