pvector: Expose non-concurrent priority vector.

pvector: Expose non-concurrent priority vector.

PMD threads use pvectors but do not need the overhead of the
concurrent version.  Expose the non-concurrent version for
that use.

Note that struct pvector is renamed as struct cpvector (for concurrent
priority vector), and the former struct pvector_impl is now struct
pvector.

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