Better time.Duration to string conversion.

In all the mysql and sqlite and postgresql wrapper,fun toInternal in collection.go.

Reserved to the millisecond:
```
return fmt.Sprintf(TimeFormat, int(t/time.Hour), int(t/time.Minute%60), int(t/time.Second%60), t%time.Second/time.Millisecond)
```

Fixing db.Fields for MongoDB, related: https://groups.google.com/forum/?fromgroups=#!topic/gosexy/3QcW40qZYfk

sqlite: Benchmark applying BEGIN and END transaction optimizations