TNonblocking server should release handler as soon as connection closes

Because connections are cashed until new connection is accepted, handlers from *IfFactory are not released when connection closes.

This is a little inconvenient if for example, different handlers are used per user to hold some client session informations.

Change includes single line added in close - reset of processor_ shared_ptr. This pointer is later assigned in init (when connection is reused) so there shouldn't be any performance change.