Basically I followed the instructions from https://github.com/yyuu/jetty-nosql-memcached with minor tweaks.
etc/jetty.xml
<Set name="sessionIdManager">
<New id="memcachedSessionIdManager" class="org.eclipse.jetty.nosql.memcached.MemcachedSessionIdManager">
<Arg><Ref id="Server" /></Arg>
<Set name="serverString">localhost:11211</Set>
<Set name="keyPrefix">session:</Set>
</New>
</Set>
<Call name="setAttribute">
<Arg>memcachedSessionIdManager</Arg>
<Arg><Ref id="memcachedSessionIdManager" /></Arg>
</Call>
contexts/uutiset.xml
  <Ref name="Server" id="Server">
    <Call id="sessionIdManager" name="getAttribute">
      <Arg>memcachedSessionIdManager</Arg>
    </Call>
  </Ref>
  <Set name="sessionHandler">
    <New class="org.eclipse.jetty.server.session.SessionHandler">
      <Arg>
        <New id="memcachedSessionManager" class="org.eclipse.jetty.nosql.memcached.MemcachedSessionManager">
          <Set name="sessionIdManager">
            <Ref id="sessionIdManager" />
          </Set>
        </New>
      </Arg>
    </New>
  </Set>