net: tcp: Allow to explicitly manage TCP receive window

This fixes the existing situation that "if application buffers data,
it's the problem of application". It's actually the problem of the
stack, as it doesn't allow application to control receive window,
and without this control, any buffer will overflow, peer packets
will be dropped, peer won't receive acks for them, and will employ
exponential backoff, the connection will crawl to a halt.

This patch adds net_context_tcp_recved() function which an
application must explicitly call when it *processes* data, to
advance receive window.

Jira: ZEP-1999

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>