[DCCP] Give precedence to the biggest ELAPSED_TIME

We can get this value in an TIMESTAMP_ECHO and/or in an ELAPSED_TIME option, if
receiving both give precendence to the biggest one.

In my tests they are very close if not equal at all times, so we may well think
about removing the code in CCID3 that inserts this option and leaving this to
the core, and perhaps even use just TIMESTAMP_ECHO including the elapsed time.

Signed-off-by: Arnaldo Carvalho de Melo <acme@mandriva.com>