In this commit, we update the SQLStore's ForEachNodeCacheable and
ForEachNodeCached methods to use batch collection for node feature bits.
This results in the following performance gains:
```
name old time/op new time/op delta
ForEachNodeCacheable-native-sqlite-10 184ms ± 2% 145ms ±10% -21.45% (p=0.000 n=10+10)
ForEachNodeCacheable-native-postgres-10 697ms ± 8% 51ms ± 4% -92.68% (p=0.000 n=9+10)
```
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