ofproto: Revertible eviction.

ofproto: Revertible eviction.

Handling evictions was broken in the previous patches. Eviction took
place early in the commit, and actually inappropriately bumped the
version number too early.  Now eviction is treated much like a flow
modification, where a new rule replaces the old one, but just without
any 'inheritance' from the evicted rule to the new rule.  This makes
evictions to be executed only when commit is successful, as evictions
are reverted like any other changes when the commit fails.

Signed-off-by: Jarno Rajahalme <jrajahalme@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>