net: tcp: Add exponential backoff for ZWP probing

Persist timer should implement exponential backoff, as per RFC 1122:

    The transmitting host SHOULD send the first zero-window
    probe when a zero window has existed for the retransmission
    timeout period (see Section 4.2.2.15), and SHOULD increase
    exponentially the interval between successive probes.

Implement this, by following Linux behaviour, and simply double the
timeout or each probe transmission.

Additionally, prevent reseting the persist timer in case an
acknowledgment is received with zero window size, and the timer is
already running.

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