ofproto: Add global locking around flow table changes.

ofproto: Add global locking around flow table changes.

This makes 'ofproto_mutex' protect the flow table well enough that threads
other than the main one can realistically modify flows.

I need to look at the interface between ofproto and connmgr: I think that
there might need to be some locking there too.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Ethan Jackson <ethan@nicira.com>