Enabling configure TLS sessions.

To support TLS sessions, Unit uses the OpenSSL built-in session cache; the
cache_size option defines the number sessions to store.  To disable the feather,
the option must be zero.

docs/changes.xml

@@ -31,6 +31,12 @@ NGINX Unit updated to 1.25.0.
          date="" time=""
          packager="Andrei Belov <defan@nginx.com>">
 
+<change type="feature">
+<para>
+TLS sessions cache.
+</para>
+</change>
+
 <change type="feature">
 <para>
 process and thread lifecycle hooks in Ruby.