net: tcp: Implement persistent timer for sending ZWP

Instead of sending ZWP from send context, when it is detected that
window is full due to zero-window, implement a proper persistent timer,
that is scheduled once zero-window is detected. The timer is responsible
for sending ZWP to the peer and is canceled once non-zero-window is
notified by the peer.

Additionally, in case peer reported zero-window, do not trigger
retransmission from net_tcp_queue_data(), as it won't be transmitted
anyway by the stack.

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