Separating ARP/ND cache maintenance from sending gratuitous ARP and UNAs

[htibosch]

Description

In FreeRTOS+TCP, the sending of gratuitous packets was always done from the cache check routine. This PR will separate the two. Also the timing of sending gratuitous (and UNA) packets should have a random start in order to avoid IP-storms at boot time.

In the new release, I want to do this for both ARP and ND. This is how it works now:

    void vARPAgeCache()
    {
        Check the ARP cache table
        if( timer done )
        {
            reset timer;
            vARPSendGratuitous();
        }
    }

I want to use intervals for calling 'vARPSendGratuitous()' that have a random factor:

Interval = 40 seconds + ( rand() % 40 );  // Get a period between 40 and 80 seconds

And the new solution, one extra timer:

/** @brief ARP timer, to check its table entries. */
    static IPTimer_t xARPTimer;

/** @brief Gratuitous timer, to send our IP-address unsolicited. */
    static IPTimer_t xGratuitousTimer;

    void vARPAgeCache()  // triggered by xARPTimer
    {
        Check the ARP cache table
    }
    
    void vARPSendGratuitous() // triggered by xGratuitousTimer
    {
        Send a gratuitous packet to advertise my IP
    }

Test Steps

Build a project, run it, and play with the value of:
ipARP_TIMER_PERIOD_MS
ipND_TIMER_PERIOD_MS

Checklist:

• I have tested my changes. No regression in existing tests.
• I have modified and/or added unit-tests to cover the code changes in this Pull Request.

Related Issue

Before this PR, gratuitous ARPs are sent at unpredictable times.

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[george-palmsens]

Hi - Is this relevant to #1341?