only_run_bfd experiment.

After creating tunnels and enabling bfd sessions,
use "ovs-appctl only-run-bfd" to enable the mode which forces program
to skip all bfd unrelated functions.

Right now, this commit only works with single-sided bfd session, meaning
there is no corresponding peer seesion. This is in that, the commit skips
the execution of bridge_run_fast(), which will handle the receving of bfd
control messages. Including the bridge_run_fast() function means a lot of
unnecessary codes executed.

To solve this issue requires more delicate modification, that is, moving
the handler function of upcalls to ofproto_run().