net: tcp: Move TCP Kconfig options to separate file

The number of Kconfig options for the TCP stack grew considerably,
therefore it makes sense to move them to a separate file not to bloat
the Kconfig file with generic networking options.

Take this opportunity to reorder TCP options, so that protocol
parameters (timings/buffer sizes) are not mixed up with optional
protocol features (fast retransmit/congestion avoidance etc.).

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