OFStateManager: throttle flow expiration

When a flow is expired it may result in sending an async message to the
controller(s). If too many flows are expired in a short period of time the
connection write queue will overflow and lead to drops. Fix this by using a
simple timer instead of the long running task, limiting the expiration rate to
around 30/s.
Merge into master from pull request #148:
OFStateManager: throttle flow expiration (https://github.com/floodlight/indigo/pull/148)