BFD: Decreasing minimal transmit and receive interval

I found the BFD transmit interval was lowerbounded by the default value
without warning, although documentation does not consider a lowerbound.

Testing has been performed with transmit and receive intervals as low as 1
ms, and although CPU overhead was effected (especially with multiple BFD
sessions such as 6 and higher), it worked well.

Signed-off-by: Niels van Adrichem <n.l.m.vanadrichem@tudelft.nl>
Signed-off-by: Ben Pfaff <blp@nicira.com>