Implement basic multiple table support.

Implement basic multiple table support.

This implements basic multiple table support in ofproto and supporting
libraries and utilities. The design is the same as the one that has been
on the Open vSwitch "wdp" branch for a long time.  There is no support for
multiple tables in the software switch implementation (ofproto-dpif), only
a set of hooks for other switch implementations to use.

To allow controllers to add flows in a particular table, Open vSwitch adds
an OpenFlow 1.0 extension called NXT_FLOW_MOD_TABLE_ID.