ofproto: Start ofport allocation from the previous max after restart.

ofproto: Start ofport allocation from the previous max after restart.

We currently do not recycle ofport numbers from interfaces that are recently
deleted by maintaining the maximum allocated ofport value and
allocating new ofport numbers greater than the previous maximum.
But after a restart of ovs-vswitchd, we start again from ofport value of '1'.
This means that after a restart, we can immeditaley recycle the 'ofport'
value of the most recently deleted interface.

With this commit, during ovs-vswitchd initial configuration, we figure
out the previously allocated max ofport value. New interfaces get ofport
value that is greater than this.

Signed-off-by: Gurucharan Shetty <gshetty@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>