Add mechanism to set OpenSSL session timeout

In a sofia profile, you can now set the parameter tls-timeout to a
positive integer value which represents the maximum time in seconds
that OpenSSL will keep a TLS session (and its ephemeral keys) alive.

This value is passed to OpenSSL's SSL_CTX_set_timeout(3).

OpenSSL's default value is 300 seconds, but the relevant standard
(RFC 2246) suggests that much longer session lifetimes are
acceptable (it recommends values less than 24 hours).

Longer values can be useful for extending battery life on mobile
devices.

Signed-off-by: Travis Cross <tc@traviscross.com>

src/mod/endpoints/mod_sofia/conf/sofia.conf.xml

@@ -242,6 +242,8 @@
         <!-- TLS version ("sslv23" (default), "tlsv1"). NOTE: Phones may not
              work with TLSv1 -->
         <param name="tls-version" value="$${sip_tls_version}"/>
+        <!-- TLS maximum session lifetime -->
+        <!-- <param name="tls-timeout" value="300"/> -->
 
         <!-- turn on auto-flush during bridge (skip timer sleep when the socket
              already has data) (reduces delay on latent connections default