OpenFlow 1.4 introduces the ability to turn on flow table eviction with an
OFPT_TABLE_MOD message specifying OFPTC_EVICTION. It also adds related
machinery to other messages that mention OFPTC_* fields. This commit adds
support for the new feature, implementing it as a second, parallel way to
enable flow table eviction. It takes more work than it seems like it
should because there is so much weirdness with the treatment of OFPTC_*
flags over the evolution of OpenFlow; please refer to the explanation in
DESIGN.md for more information.
This commit also adds related support to ovs-ofctl, plus tests.