OFConnectionManager: fix disconnection of listen connections

When disconnection was changed to operate on controllers instead of individual
connections, listen connections were broken because they aren't really
associated with a controller. These changes fix this by creating a new
controller for each accepted connection.
OFConnectionManager: add ephemeral bit

This adds a new field that controls whether the controller will be removed on
disconnect. This is less error-prone than overloading the "listen" bit.
Merge into master from pull request #98:
OFConnectionManager: fix disconnection of listen connections (https://github.com/floodlight/indigo/pull/98)