net: tcp: Keep track of recv window size change since last ACK

Windows TCP stack has a peculiar behavior - when running iperf, it will
fill out the RX window almost entirely, but will not set PSH flag on
packets. In result, our stack would delay the ACK and thus window
update, affecting throughputs heavily.

In order to avoid that, keep track of the most recent window size
reported to the peer, and reduce it when receiving new data. In case the
RX window, as seen from the peer perspective, drops below certain
threshold, and the real RX window is currently empty, send an ACK
immediately when updating window, so that peer can continue
with sending data.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>
(cherry picked from commit 349bf81e)