caddytls: Mark storage clean timestamp at end of routine

[mholt]

See discussion on 42b7134

/cc @Gr33nbl00d

[Gr33nbl00d]

I think that should do it,
Only remark is what will happen if there is a panic during cleanup. Not sure we would need to handle this case.
But if we define a defer function after the condition which actually would set the finishTime we could avoid multiple fast retrys on panic.

[mholt]

I don't think multiple fast retries will be a concern here. I could be wrong, but I'd rather wait and see if that's actually the case because timers still call this function.