OFStateManager: convert aggregate stats to a long running task

Essentially the same as the flow stats handler.
OFStateManager: fix leak in ind_core_aggregate_stats_request_handler
OFStateManager: convert non-strict flow-modify to a long running task
OFStateManager: add barrier tracking to unit test

Both flow-delete and flow-modify now spawn long running tasks, so we can't rely
on the OFStateManager message handler returning to mean the operation is
complete.
Merge into master from pull request #33:
convert flow modify and aggregate stats to long running tasks (https://github.com/floodlight/indigo/pull/33)