ovsdb replication: Provide option to configure probe interval.

ovsdb replication: Provide option to configure probe interval.

When ovsdb-server is in backup mode and connects to the active
ovsdb-server for replication, and if takes more than 5 seconds to
get the dump of the whole database, it will drop the connection
soon after as the default probe interval is 5 seconds. This
results in a snowball effect of reconnections to the active
ovsdb-server.

This patch handles or mitigates this issue by setting the
default probe interval value to 60 seconds and provide the option to
configure this value from the unixctl command.

Other option could be increase the value of 'RECONNECT_DEFAULT_PROBE_INTERVAL'
to a higher value.

Acked-by: Mark Michelson <mmichels@redhat.com>
Acked-by: Dumitru Ceara <dceara@redhat.com>
Signed-off-by: Numan Siddique <numans@ovn.org>
Signed-off-by: Ben Pfaff <blp@ovn.org>