Certificate cache TTL

[hazcod]

Hi! Thank you for the project.

I've noticed that the generated MITM certificates are never purged from memory.
So in case of a long running instance of martian, the memory usage would only increase.

I think it would be nice to be able to use the Certificate Validity (SetValidity) as a TTL on the certificate map key.

[bramhaghosh]

This seems reasonable - I'd be happy to review a PR that adds in this behavior.