Add buffer size parameter

[bboreham]

On a busy machine the default socket buffer size can be overflowed by conntrack messages, so give the caller the option to use a bigger buffer.

I realise the change in API may be unwelcome - happy to code it differently if you prefer.

[alicebob]

Thanks for the PR!

I see the problem, and this solution makes sense, but I like the simplicity of the current methods, such as:
func Connections() ([]Conn, error)

Would there be a way to keep them as they are and still solve your issue?

[bboreham]

I've redone it with extra APIs ConnectionsSize() and FollowSize().
Let me know what you think.

[alicebob]

great, thanks!