Timed out flows not removed from operational space

Flows that timed (or were deleted) from OpenFlow nodes do not get deleted from hte controller's operational space. Once a flow is added, it stays there until the its node is deleted.

To reproduce:

• Add flows with a small hard timeout
• Let one or two stats collection cycles finish
• Wait for the 'timeout' seconds
• Let one or two stats collection cycles finish
• The table stats get reported correctly (i.e. the flows disappear), but the flow stats are still being reported.