net: lib: http_server: Move stream-specific flags to stream context

The information about replied headers or END_OF_STREAM flag are
stream-specific and not general for a client. Hence, need to move them
to the stream context.

For the upgrade case, we need to allocate a new stream now when HTTP1
request /w upgrade field is received. The stream ID in such case is
assumed to be 1 according to RFC.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>