ofproto: Make ->construct() and ->destruct() more symmetrical.

ofproto: Make ->construct() and ->destruct() more symmetrical.

The ofproto_provider's ->construct() was required to allocate and
initialize the flow tables, but ->destruct() was not allowed to
uninitialize and free them.  This arrangement is oddly asymmetrical (and
undocumented), so this commit changes the code so that the client is
responsible for both allocation and freeing.

Reported-by: Hao Zheng <hzheng@nicira.com>
CC: Hao Zheng <hzheng@nicira.com>