datapath: Add commentary to conntrack.c

datapath: Add commentary to conntrack.c

Upstream commit:
    commit 9f13ded8d3c715147c4759f937cfb712c185ca13
    Author: Jarno Rajahalme <jarno@ovn.org>
    Date:   Thu Mar 10 10:54:18 2016 -0800

    openvswitch: Add commentary to conntrack.c

    This makes the code easier to understand and the following patches
    more focused.

    Signed-off-by: Jarno Rajahalme <jarno@ovn.org>
    Acked-by: Joe Stringer <joe@ovn.org>
    Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Jarno Rajahalme <jarno@ovn.org>
Acked-by: Jesse Gross <jesse@kernel.org>