Create request object only after the first byte was received.

Create request object only after the first byte was received.

Previously, we always created an object and logged 400 (Bad Request)
in access log if a client closed connection without sending any data.
Such a connection was counted as "reading".

Since it's common for modern browsers to behave like this, it's no
longer considered an error if a client closes connection without
sending any data, and such a connection will be counted as "waiting".

Now, we do not log 400 (Bad Request) and keep memory footprint as
small as possible.