OpenFlow says:

"OpenFlow messages are processed out of order (see 6.2), and the
processing of some requests by the switch may take a long time,
therefore a controller must never terminate a connection because a
request is taking too much time. An exception to that rule is for echo
replies, a controller or a switch may terminate a connection if the
reply for an echo request it sent takes too much time, however there
must be a way to disable that feature and the timeout should be large
enough to accommodate a wide variety of conditions."

A maximum of 4 seconds (1 lifetime + 3 echo) is not much if the OpenFlow
connection is conguested due to a large amount of messages to process,
so double the echo timout.

Since rofl-common also sends out echo requests on long-standing barrier
requests, but barrier-requests inhibit processing messages until
completed, also increase the lifetime check significantly to allow
barrier_requests to take up to 16 seconds, not 4.

This should significantly increase the stability of the OpenFlow
connection under stress conditions.

Signed-off-by: Jonas Gorski <jonas.gorski@bisdn.de>

Allow configuring two OpenFlow connection timeouts:

* of_timeout_echo: timeout of sent echo requests
* of_timeout_lifecheck: delay of sending echo requests if rx is idle

This allows overriding the defaults in case the selected values are too
tight.

Signed-off-by: Jonas Gorski <jonas.gorski@bisdn.de>