MXS-1669: Fix load average tracking
The output of `show threads` could have a negative historic thread load average that could be explained by the overflow of the signed 32-bit integer used to count the number of samples. The time that each thread started to process an event for a DCB used an old value that is no longer used. Updating this to DCB::last_read retains the 2.0 behavior.
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		@ -125,7 +125,7 @@ SPINLOCK pollqlock = SPINLOCK_INIT;
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 * poll completion, a value of 1 or less is the ideal.
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 */
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static double load_average = 0.0;
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static int load_samples = 0;
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static uint64_t load_samples = 0;
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static int load_nfds = 0;
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static double current_avg = 0.0;
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static double *avg_samples = NULL;
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@ -755,7 +755,7 @@ poll_waitevents(void *arg)
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            pollStats.n_fds[(nfds < MAXNFDS ? (nfds - 1) : MAXNFDS - 1)]++;
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            load_average = (load_average * load_samples + nfds) / (load_samples + 1);
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            atomic_add(&load_samples, 1);
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            atomic_add_uint64(&load_samples, 1);
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            atomic_add(&load_nfds, nfds);
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            /*
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@ -1368,7 +1368,7 @@ dShowThreads(DCB *dcb)
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            dcb_printf(dcb,
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                       " %2d | %-10s | %6d | %-16p | <%3lu00ms | %s\n",
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                       i, state, thread_data[i].n_fds,
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                       thread_data[i].cur_dcb, 1 + hkheartbeat - dcb->evq.started,
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                       thread_data[i].cur_dcb, 1 + hkheartbeat - dcb->last_read,
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                       event_string);
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            if (from_heap)
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@ -1388,8 +1388,8 @@ dShowThreads(DCB *dcb)
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static void
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poll_loadav(void *data)
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{
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    static  int last_samples = 0, last_nfds = 0;
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    int new_samples, new_nfds;
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    static uint64_t last_samples = 0, last_nfds = 0;
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    uint64_t new_samples, new_nfds;
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    new_samples = load_samples - last_samples;
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    new_nfds = load_nfds - last_nfds;
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