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)
OFConnectionManager: add notifications for barriers

Some modules may wish to defer work until a barrier is received. This new
interface allows them to register to be notify when a (blocked) barrier arrives
on a connection. The module should then decide whether it holds a barrier
blocker on that connection, and if so finish its work and unblock the barrier.
minimatch: optimize minimatch_more_specific for empty query
OFStateManager: process multiple flowtable entries between calls to ind_soc_should_yield

ind_soc_should_yield() was taking 30% of CPU time in my benchmarks.
Merge into master from pull request #158:
OFConnectionManager: add notifications for barriers (https://github.com/floodlight/indigo/pull/158)
Merge into master from pull request #159:
Optimize flowtable iteration (https://github.com/floodlight/indigo/pull/159)
OFConnectionManager: add assertions for clang static analyzer

cxn_id_to_connection could return NULL, but it should not in these cases.
Merge into master from pull request #160:
OFConnectionManager: add assertions for clang static analyzer (https://github.com/floodlight/indigo/pull/160)