cmap implements duplicates as linked lists, which causes removal of
rules to become (O^2) with large number of duplicates.  This patch
fixes the problem by introducing a new 'counting' variant of the cmap
(ccmap), which can be efficiently used to keep counts of inserted hash
values provided by the caller.  This does not require a node in the
user data structure, so this makes the user implementation a bit more
memory efficient, too.

Signed-off-by: Jarno Rajahalme <jarno@ovn.org>
Acked-by: Ben Pfaff <blp@ovn.org>