Use SSL_session_reused API

[adiroiban]

I guess that this should be OpenSSL.SSL.Connection.session_reused()

It should return the number 1 or 0 to keep it as close a possible to the OpenSSL API.

It should not return a boolean.

I see that OpenSSL.SSL.Connection.renegotiate_pending() return a boolean, but SSL_renegotiate_pending returns an int.

[adiroiban]

I'm working on a PR for this