Fix BUG#47: Add simple controller failover to provide control plane
session redundancy.
Improved BUG#47: Add liveliness check of a peer using ECHO exchange
with non-zero XID.
Fix BUG#50: Change minimum configurable value of inactivity-timer for
OpenFlow control plane session to provide quick controller failover.
Change ofprotocol(8) man page to describe failover.
