ofproto meters: Keep provider meter_id over mods.

Changed the ofproto meter API to require the provider keep the provider
meter ID unchanged when modifying a meter.  This makes the mapping from
an OpenFlow meter ID to a datapath meter ID consistent over the lifetime
of the meter.

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