ofproto-dpif: Enable NXAST_SAMPLE only if the datapath supports it.

This prevents using an older datapath from breaking forwarding.

CC: Romain Lenglet <rlenglet@vmware.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>
bfd: Make bfd decay test robust.

With ovs multithreading implementation, the bfd decay test
becomes fragile due to its high dependency on timing sequence.
This commit removes these dependencies and makes the test robust.

Signed-off-by: Alex Wang <alexw@nicira.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>