net: tcp: Prevent Silly Window Syndrome

Implement a mechanism, according to RFC 813, which allows to prevent so
called "Silly Window Syndrome" - a scenario where the TCP receiver keeps
reporting small window sizes in the acknowledgments, effectively
limiting the connection throughput. This allows to improve performance
in low-buffer configurations, where the maximum window size is small,
and the issue was hitting quite often.

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