Optimise chunking in OfAgent and parametrize chunk length

Currently (after https://gerrit.opencord.org/#/c/19023/) the ofagent supports chunking of OF messages (multipart) with the REPLY_MORE flag in a very crude manner, a static content of 500 for ports and 150 for flows has been hardcoded into the stats.go class.
This tasks aims at removing that hardcoded value, making it configurable and also doing some analysis of a better number according to the 64kb max length of the OF message.
Also a better chunking algorithm could probably be put in place.