connmgr: Use 'ofproto_mutex' to protect ofconns from being destroyed.

connmgr: Use 'ofproto_mutex' to protect ofconns from being destroyed.

Code in the ofproto-dpif miss handler threads can currently access
ofconns, sending flow_removed and flow monitor messages due to NXAST_LEARN
actions.  Nothing currently protects those threads from accessing ofconns
that are in the process of being destroyed.  This commit adds protection
'ofproto_mutex', which NXAST_LEARN already takes.

Later patches will address other races that remain.

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