To detect connection_t staleness, add generation_id and utility routines
to convert a connection_t* to a void* cookie.

Use this void* cookie for OF object tracking, to prevent crashes when
an OF object associated with a stale connection is deleted.

Convert flow_stats handler to a long running task.

Increase WRITE_BUFFER_SIZE to handle a 129k flow stats reply.
Adjust log levels.
Do not change WRITE_BUFFER_SIZE.
Use uintptr_t for cookie manipulation.
Missing cast.
Merge into master from pull request #27:
flow-stats-reply handling rework (https://github.com/floodlight/indigo/pull/27)