OFConnectionManager: factor out outstanding_op_count manipulation
OFConnectionManager: barrier blocker interface
OFStateManager: replace ind_core_dup_tracking with barrier blockers

The only long running task that actually used the message contents was flow
modify. All the others only need the OpenFlow version to send the reply with,
and the XID.
OFStateManager: fix barrier handling in utest

Replace LOCI message tracking with barrier blockers.
OFConnectionManager: ensure connection cookie is not NULL

Also added some comments.
OFConnectionManager: remove ind_cxn_message_track_setup
OFStateManager: add connection ID to table ops

This is necessary for the table implementations to use barrier blockers, since
they require the connection ID.
Merge remote-tracking branch 'origin/master' into barrier-blockers

Conflicts:
	modules/OFStateManager/module/src/handlers.c
OFConnectionManager: add NULL-avoidance comments to cxn cookie
indigo: add comment to barrier blocker interface
indigo: wrap long lines
Merge into master from pull request #109:
Barrier blockers (https://github.com/floodlight/indigo/pull/109)