Session cache opts

[bak1an]

It might be useful to get/set session cache timeout and session cache size. Openssl provides us with https://www.openssl.org/docs/ssl/SSL_CTX_sess_set_cache_size.html and https://www.openssl.org/docs/ssl/SSL_CTX_set_timeout.html to perform this.

This pull requests adds above methods to Ctx.

[jtolio]

this is great! would it be alright to request that the timeout value type be changed to time.Duration instead of int? that way library users don't need to check the docs for time units

[bak1an]

@jtolds sounds totally reasonable.

I'm going to implement this later this evening. Will update this pull request when finished.

[bak1an]

@jtolds done

[jtolio]

thanks!