bfd: Send FINAL immediately after receiving POLL.

Commit 307464a11 (ofproto-dpif-monitor: Use heap to order the mport
wakeup time.) makes bfd only send packet at specified periodic instant.
This fails to meet the RFC5880 requirement, which requires bfd send
FINAL immediately after receiving POLL.

This commit fixes the above issue by scheduling bfd to send FINAL
within 100 ms after receiving POLL.

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