OFConnectionManager: add generation count to indigo_cxn_id_t
OFConnectionManager: replace connection cookies with IDs

The connection ID now includes the same generation number the cookie did.
OFConnectionManager: add pause/resume APIs

OFStateManager will use these interfaces to pause reading messages from a
connection while a long-running task is in flight.
OFStateManager: pause connection while handling long-running tasks

Pausing the connection also prevents later barrier requests from being handled,
so the barrier blockers are redundant.
OFConnectionManager: remove unused connection generation_id field
Merge into master from pull request #157:
Add generation ID to the connection ID, and pause reading messages during long-running tasks (https://github.com/floodlight/indigo/pull/157)