sc-inc-gpc0() doesn't update the gpc0_rate

[Ruoshan Huang]

hi,
    HAProxy (1.6.3) doesn’t update the gpc0_rate accordingly when I do things 
like:

```
    stick-table type ip size 1k expire 30m store gpc0_rate(10s),gpc0
    tcp-request inspect-delay 3s
    tcp-request connection track-sc0 src
    tcp-request connection sc-inc-gpc0(0)
```

the gpc0 did get increased, but the gpc0_rate was NOT updated. the function 
`action_inc_gpc0` just do `gpc0++` and no more.

   I could use the `acl increase_gpc sc_inc_gpc0(0) gt 0` instead, as the 
function `smp_fetch_sc_inc_gpc0` is called (so the gpc0_rate is updated).

   Is this a bug?

--
Good day!
ruoshan