Rework meterpreter SSL & pass datastore to handle_connection()

This allows HandlerSSLCert to be used to pass a SSL certificate into the Meterpreter handler. The datastore has to be passed into handle_connection() for this to work, as SSL needs to be initialized on Session.new. This still doesn't pass the datastore into Meterpreter directly, but allows the Session::Meterpreter code to extract and pass down the :ssl_cert option if it was specified. This also fixes SSL certificate caching by expiring the cached cert from the class variables if the configuration has changed. A final change is to create a new SSL SessionID for each connection versus reusing the SSL context, which is incorrect and may lead to problems in the future (if not already).