net: tcp: Rework data queueing API

Rework how data is queued for the TCP connections:
  * net_context no longer allocates net_pkt for TCP connections. This
    was not only inefficient (net_context has no knowledge of the TX
    window size), but also error-prone in certain configuration (for
    example when IP fragmentation was enabled, net_context may attempt
    to allocate enormous packet, instead of let the data be fragmented
    for the TCP stream.
  * Instead, implement already defined `net_tcp_queue()` API, which
    takes raw buffer and length. This allows to take TX window into
    account and also better manage the allocated net_buf's (like for
    example avoid allocation if there's still room in the buffer). In
    result, the TCP stack will not only no longer exceed the TX window,
    but also prevent empty gaps in allocated net_buf's, which should
    lead to less out-of-mem issues with the stack.
  * As net_pkt-based `net_tcp_queue_data()` is no longer in use, it was
    removed.

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