ivs: increase SocketManager socket limit

We use a few sockets per port (though not all are registered with
SocketManager), so the limit of 1024 was a scalability problem.
ivs: increase RLIMIT_NOFILE to match SOCKETMANAGER_CONFIG_MAX_SOCKETS
Merge into master from pull request #207:
increase maximum number of file descriptors to 4096 (https://github.com/floodlight/ivs/pull/207)