Better threading model for SwitchHandler implementation

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