Implement OpenFlow 1.4+ OFPTC_EVICTION.

Implement OpenFlow 1.4+ OFPTC_EVICTION.

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.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Co-authored-by: Saloni Jain <saloni.jain@tcs.com>
Signed-off-by: Saloni Jain <saloni.jain@tcs.com>
Acked-by: Jarno Rajahalme <jrajahalme@nicira.com>