Use new method to describe table entries in OpenFlow wire protocol.

Older versions of OpenFlow pre-defined a few different types of tables and the
Features Reply message would indicate how many of those entries existed.  This
was very inflexible, so now the Features Reply just indicates how many tables
there are.  The controller now sends a Table Stats request to find out how
many entries are supported and the fields that can be wildcarded.

This change also removes the "buffer_mb" field from the Features Reply, since
it was not being used and seemed to be of limited value.