keep-alive monitor

once connection is being putted to keep-alive cache we start keep-alive monitor - just read some data to the dummy buffer.
if connection is closed by the server side during idle period, then keep-alive monitor ends with EndOfFile error code and connection will be closed and removed.
all of these is because Connection::is_alive() doesn't work properly - this method was removed.

we stop keep-alive monitor just before get connection from the keep-alive cache.