iOS: 20min `~poll` messages cause server spikes and thundering herd

[binwiederhier]

Number of GETs during the peak minutes:

$ cat /var/log/nginx/access.log|grep -E GET |awk '{print $4}'| cut -d: -f 2,3 | uniq -c|less
    190 00:15
    183 00:16
    650 00:17 <<<
    180 00:18
    231 00:19
...
    197 00:34
    213 00:35
    152 00:36
    660 00:37  <<<
    180 00:38
    273 00:39
    198 00:40
...
    233 00:55
    205 00:56
    686 00:57  <<<
    183 00:58
    233 00:59
    273 01:00

[binwiederhier]

Dup of #677