ofproto-dpif: Configure datapath max-idle through ovs-vsctl.

This patch adds a new configuration option, "max-idle" to the
Open_vSwitch "other-config" column. This sets how long datapath flows
are cached in the datapath before ovs-vswitchd thread expire them.

This commit is a backport of commit 72310b04 (upcall: Configure
datapath max-idle through ovs-vsctl.).

Signed-off-by: Alex Wang <alexw@nicira.com>
Acked-by: Joe Stringer <joestringer@nicira.com>