Http1.1. Add persistent connection support

[sgrebnov]

Right now we always close connection after sending response (we also always put Connection: close header to the response).

We may want to review current implementation by re-using connection in case of request contains Connection: keep-alive header

[Tratcher]

Connection: Keep-alive is for HTTP/1.0. In HTTP/1.1 Keep-Alive is the default (does not need to be sent), but instead the client/server sends Connection: Close if they do not want keep-alive.