ct-dpif, dpif-netlink: Add conntrack timeout policy support

ct-dpif, dpif-netlink: Add conntrack timeout policy support

This patch first defines the dpif interface for a datapath to support
adding, deleting, getting and dumping conntrack timeout policy.
The timeout policy is identified by a 4 bytes unsigned integer in
datapath, and it currently support timeout for TCP, UDP, and ICMP
protocols.

Moreover, this patch provides the implementation for Linux kernel
datapath in dpif-netlink.

In Linux kernel, the timeout policy is maintained per L3/L4 protocol,
and it is identified by 32 bytes null terminated string.  On the other
hand, in vswitchd, the timeout policy is a generic one that consists of
all the supported L4 protocols.  Therefore, one of the main task in
dpif-netlink is to break down the generic timeout policy into 6
sub policies (ipv4 tcp, udp, icmp, and ipv6 tcp, udp, icmp),
and push down the configuration using the netlink API in
netlink-conntrack.c.

This patch also adds missing symbols in the windows datapath so
that the build on windows can pass.

Appveyor CI:
* https://ci.appveyor.com/project/YiHungWei/ovs/builds/26387754

Signed-off-by: Yi-Hung Wei <yihung.wei@gmail.com>
Acked-by: Alin Gabriel Serdean <aserdean@ovn.org>
Signed-off-by: Justin Pettit <jpettit@ovn.org>