fix: query mising

[qingyunha]

当cache中缺失点时， 可能导致后续的点的值全为NaN，直到rrd再次刷新，有半个小时的时间

[laiwei]

cache中为什么会出现缺失？哪种情况下会出现哈

[qingyunha]

不清楚，我们交换机端口流量出现了这样的情况，半个小时没图，我也确实看到了cached有不连续的点

[qingyunha]

这是一次查询，从前两次的结果可以看出它们中间缺了一个点...

cached  items : 10
rrdfile items : 7

format cached items......
fomat  0
itemIdx < items_size-1 : true
ts == items[itemIdx].Timestamp : true
ts == items[itemIdx+1].Timestamp-int64(step) : false
COUNTER TYPE Missing??

fomat  0
itemIdx < items_size-1 : true
ts == items[itemIdx].Timestamp : false
ts == items[itemIdx+1].Timestamp-int64(step) : true
COUNTER TYPE Missing??

fomat  0
itemIdx < items_size-1 : true
ts == items[itemIdx].Timestamp : false
ts == items[itemIdx+1].Timestamp-int64(step) : false
COUNTER TYPE Missing??

fomat  0
itemIdx < items_size-1 : true
ts == items[itemIdx].Timestamp : false
ts == items[itemIdx+1].Timestamp-int64(step) : false
COUNTER TYPE Missing??

fomat  0
itemIdx < items_size-1 : true
ts == items[itemIdx].Timestamp : false
ts == items[itemIdx+1].Timestamp-int64(step) : false
COUNTER TYPE Missing??

fomat  0
itemIdx < items_size-1 : true
ts == items[itemIdx].Timestamp : false
ts == items[itemIdx+1].Timestamp-int64(step) : false
COUNTER TYPE Missing??

fomat  0
itemIdx < items_size-1 : true
ts == items[itemIdx].Timestamp : false
ts == items[itemIdx+1].Timestamp-int64(step) : false
COUNTER TYPE Missing??

fomat  0
itemIdx < items_size-1 : true
ts == items[itemIdx].Timestamp : false
ts == items[itemIdx+1].Timestamp-int64(step) : false
COUNTER TYPE Missing??

fomat  0
itemIdx < items_size-1 : true
ts == items[itemIdx].Timestamp : false
ts == items[itemIdx+1].Timestamp-int64(step) : false
COUNTER TYPE Missing??

fomat  0
itemIdx < items_size-1 : true
ts == items[itemIdx].Timestamp : false
ts == items[itemIdx+1].Timestamp-int64(step) : false
COUNTER TYPE Missing??

after format cached items
<RRDData:Value:NaN TS:1470123900 2016-08-02 15:45:00>
<RRDData:Value:NaN TS:1470123840 2016-08-02 15:44:00>
<RRDData:Value:NaN TS:1470123780 2016-08-02 15:43:00>
<RRDData:Value:NaN TS:1470123720 2016-08-02 15:42:00>