ofproto: Make number of packet handler threads runtime configurable.

ofproto: Make number of packet handler threads runtime configurable.

This commit adds a new column "n-handler-threads" to the Open_vSwitch
table. This is used to set the number of upcall handler threads created by
the ofproto-dpif-upcall module.

Signed-off-by: Alex Wang <alexw@nicira.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>