Current ISwitch implementation viz. SwitchHandler encapsulates the
ExecutorService for being used by outgoing packets (syncsend uses this). Mapping
each switch to corresponding ISwitch and starting ExecutorService with
fixed-threadpool-size of 4 for each switch.

This could result in following impacts

1. Upfront allocation of threads - could potentially lead to extensive
context-switching if north to south message-traffic (proactive flow pushes)
become quiet extensive
2. No central thread-pool from which switches could steal from based on demand.

It would be better if a common threadpool with which tasks could be multiplexed for all switches could be used for better resource management