"Avigilon has already approached any company with their patent portfolio. Have they? If yes then does somebody have any details?"
Avigilon has already publicly said they are reaching out to some companies. Their explanation was that they were going to start with a limited number of companies and then expand. I don't have any details about what they are asking for from 'targets' but they have said they plan to look more broadly beyond the OV patents and back towards original licensees that may not be covered for other patents.
"Of course, big guys have their own war chests and resources to fight back but what can do small & medium players to protect their customers?"
Ironically, the big guys were the ones who mostly folded when OV sued a few years ago. For example, Bosch settled, Pelco settled, Sony settled, etc.
"Meanwhile it does not look wise to invest in filing patents on our level"
New patents might be useful to you in the future, generally, but may not have any impact on Avigilon's position unless you somehow want to try to negotiate cross licensing of your patents / applications with theirs.
"it is questionable whether it makes sense to continue development in this area when it became tough"
My gut feel is the opposite. As a smaller company (presumably a specialist), if your analytics work well, you should be able to do well. At worse, Avigilon will get a cut of your revenue but that will not make or break you. This is much more of an issue for large companies that want to include analytics for free on low cost cameras.
"You are going into a very dangerous area here."
Disagree. Worse thing that happens is that Avigilon demands you pay them a few dollars per channel (probably) and, at your presumably smaller scale, it will not really make a difference.
If I was in your shoes, I'd probably just approach Avigilon, see if a reasonable deal can be signed, and then use that as tool to calm customers, partners, investors, etc.
Note: my advice to large companies would be completely different, as having Avigilon hold key patents over you is much more of a risk there.